It was the sort of question that could have flustered a less experienced teacher, coming as it did from a 10-year-old: “Miss, what is the difference between a gang bang and an orgy?”

But Linda Castor*, a primary school teacher from south London, has heard it all before. “One is consensual, the other is not,” she calmly told the Year 6 boy, who seemed satisfied with the answer.

At one time, Mrs Castor admits that she would have panicked. Having received no sex and relationship education (SRE) at school and no training at university, there were certain words and subjects she could not bring herself to utter, let alone teach.

“When I started out in the classroom, I could not even say the word `nipple’,” she remembers. “I told myself, `This is silly - it is just a word’, but I still could not say it without getting embarrassed.”

The likelihood is that Mrs Castor is far from alone. Many teachers - in keeping with traditional British reserve - feel uncomfortable talking about sex with their pupils. But the evidence suggests that this approach is not working.

Although falling, the teenage pregnancy rate in the UK is still high compared with other countries, and the incidence of sexually transmitted infections (STIs) among teenagers is on the increase. So is it time sex education became more explicit?

Mrs Castor now recognises the importance of no-holds-barred SRE and has overcome her initial reticence when talking about sex.

“I would practise saying `vagina’, `nipple’, `breast’ and `penis’ out loud on my own,” she says. “When pupils asked about pubes, I was able to say it is called pubic hair because it lies on the pubic bone. And I could tell the boys that they need to hold their penises when they pee so that it does not go everywhere.”

Such straight-talking in SRE - which starts in her school for children as young as four - has enabled pupils to make informed decisions based on facts rather than rumour, she argues.

But it took Mrs Castor almost 30 years, which included a 10-year stint as her school’s personal, social and health education (PSHE) co-ordinator, to overcome her crippling embarrassment and develop effective teaching practices.

The outlook for today’s NQTs who are asked to teach SRE looks no rosier.

Plans to introduce compulsory sex education lessons in secondary schools were dropped by Labour in the run-up to the election, so without any statutory requirements (see box below), NQTs are likely to be just as ill- equipped as Mrs Castor was 30 years ago.

Three-quarters of teachers have been given no training in sex education, despite nearly half having been asked to teach it, according to a survey by The TES in 2008. The result is an army of teachers too poorly trained to adequately equip pupils with the information they need to keep themselves safe.

A more explicit approach to risk, relationships, consent and feelings is well overdue, argues John Lloyd, policy adviser at the PSHE Association.

“Until we get a fully-trained cohort of PSHE teachers, schools are always going to have problems teaching sex and relationship education effectively,” he says.

The evidence is unequivocal. More children are having underage sex in Britain than in any other country in Europe, according to the World Health Organisation, while teenage STI rates have risen by 58 per cent in the past five years.

A multitude of socio-economic factors undoubtedly play a part, but a dearth of decent SRE does not help, according to Ofsted. A report released earlier this year by the inspectorate found that lessons about sex, relationships and health are not good enough in a quarter of schools in England. Inspectors found that teacher embarrassment and lack of knowledge were often to blame.

In the absence of formal staff training, SRE teaching ideas, resources and materials are being acquired on an ad hoc basis. Television programmes are an unlikely source. Taking tips from TV may sound like a last resort, but Dr Radha Modgil insists it is better than nothing.

As resident doctor on Channel 4‘s The Sex Education Show - a lively, informative series for teenagers filmed in secondary schools and aired over the summer - she was surprised by the range of pupils’ knowledge. Some were incredibly clued up, while others still believed it was impossible to get pregnant if they had sex in a shower.

Cue graphic images of male and female genitalia, bringing babies into the classroom and an unflinching examination of sex, pregnancy and contraception myths.

“At the moment, pupils are caught between uncomfortable lectures in anatomy at school and explicit, misleading scenes in pornography,” Dr Modgil says. “There is little that helps them navigate their relationships and feelings.”

Dr Modgil agrees that a lack of training and support for teachers is a problem, but adds that teachers also fear a backlash from parents and the media over what could be construed as an inappropriate or overly sexualised syllabus.

Alarmist newspaper stories about schools teaching sex to young children can certainly deter teachers from exploring the nitty-gritty with pupils, particularly when they have to negotiate parental sensitivities about the subject.

The Sex Education Show originally aired at 8pm, until a hostile reaction from viewers convinced schedulers to push it back by an hour. Ironically, its new programming slot excluded a large chunk of its target audience.

However, the show was well received by teachers on the hunt for material. Dr Modgil does not expect schools to adopt the “extreme naked” tactics of the show, but hopes it may trigger innovation in SRE or at least encourage schools to adopt a more open approach to the subject.

“Sex lessons need to move beyond the biological focus and into the present day,” she argues. “If teachers do not give young people the information they need, pupils will glean it from much less reliable sources, such as friends, porn or common myths.”

Other countries already take a much more matter-of-fact approach to sex education. Fewer than 1 per cent of Dutch teenagers get pregnant every year, compared with 6 per cent in England. The Netherlands’ famous culture of tolerance and pragmatism allows children to talk openly about sexuality with their parents at home, argues Lucy Emmerson, principal officer of the Sex Education Forum, which campaigns for good-quality sex and relationships education in the UK.

Dutch teachers, meanwhile, are not bound by a national curriculum and can teach pupils about sex in a wide variety of ways. For example, pupils may be invited to write up slang words for vagina, penis and sex on the whiteboard before selecting the most appropriate, or they can discuss sexuality through questioning each other from specially prepared cards.

But communicating in these ways may be hard for teachers in the UK, admits Ms Emmerson. “They probably are not used to such a culture of openness,” she says. “It is hard to create a school culture that is at odds with a society’s wider-held beliefs.”

However, international evidence suggests that getting by on the bare minimum can also backfire. Sexuality education was made compulsory in Finnish schools in 1970, but when it was downgraded to optional in 1994 - at the same time that resources for sexual health services were cut - the quality and quantity of provision declined.

Finland subsequently experienced a surge of 14 and 15-year-olds having sex, resulting in a 50 per cent increase in teenage abortions in the late 1990s. Although it may be going too far to claim a direct link, the Finnish government recognised the need to reintroduce a clear SRE policy at a national level.

As a result, a new subject area called “health” became compulsory in primary and secondary schools in 2006. It offers some year groups a minimum of 20 hours a year on SRE, and the lessons are always delivered by well-trained teachers.

Now that SRE is firmly back on the Finnish curriculum, the situation is beginning to improve: girls are waiting until they are older to have sex and are more likely to use contraception when they do, according to a 2009 study by academics at Helsinki University. The rate of teen abortions has dropped, and there has also been a small decline in the number of teenage births.

The US has also learnt the value of using informed, assertive teaching methods. California was one of the first states to reject federal funding for the nation’s abstinence-only programme. Instead, it opted for “comprehensive sexuality education”, providing factually accurate information on all choices.

Teenage pregnancies fell by 52 per cent in California between 1992 and 2005, well above the national decline of 37 per cent. The US government has now withdrawn funding for abstinence-only programmes.

Back in the UK, schools hover somewhere between a more enlightened approach and barely meeting statutory requirements, says Ms Emmerson. “Some teachers have a brilliantly frank and matter-of-fact manner,” she adds. “Others can be really put off out of embarrassment and concern over saying the wrong thing or upsetting people.”

Mrs Castor is one teacher who has successfully made the transition. After consulting with the school council, parents and governors about the content and tone of her lessons, the subject of love and attachment is now at the forefront of the primary school’s SRE curriculum.

One aspect of the lessons involves the use of a “worry box”. Pupils are encouraged to put anonymous questions in the box, which allows them to air concerns they dare not say out loud. It also gives teachers time to go away and think about how best to respond before coming back with an informed answer.

Despite her shy beginnings, Mrs Castor is now impervious to embarrassment. “I’m forever talking to Year 5s about periods, sweating, wet dreams and erections,” she says. “It is amazing what concerns them - such as whether boys can urinate and ejaculate at the same time.”

Some teachers argue that sex education is best left to parents. In an ideal world, parents would indeed allay their children’s concerns. But this is not always the case, and in the absence of parental guidance, teachers have to step up to the mark.

One Year 5 pupil asked Mrs Castor what “cunt” meant. He had overheard his neighbour using the word and his mother was too embarrassed to give him an explanation. Mrs Castor told him it was slang for female genitalia and nothing more needed to be said.

But although Mrs Castor is open with her pupils, the constant fear of a possible backlash means she would only speak to The TES Magazine on condition of anonymity. Not surprisingly, many teachers are aware of the potential vilification they could face if they are associated with suggestions that sex education should be more explicit.

Peter Bone, head of PSHE at Fernwood School in Nottingham, is an exception in that he is happy to be publicly identified. He says SRE should be focused on keeping pupils safe by giving them the skills and confidence to resist pressure and stay in control of any situation, rather than making sure feathers stay unruffled.

And if a teacher at Fernwood is uncomfortable with any of the SRE material, he adds, they can team-teach with a PSHE specialist or the school nurse. “It is not a question of being graphic or prudish, it is a question of teaching pupils what is appropriate to their age and stage,” he says. “We may presume that a 12-year-old knows the basics, but I have learnt it is dangerous to assume. When a pupil asks if cling film is an effective form of contraception, it needs to be taken seriously.”

Once Mr Bone and his team have established ground rules, such as “no personal questions”, they field queries about masturbation, oral sex and flavoured condoms, among other topics. “The thing every pupil is after is clarity and accuracy,” Mr Bone says. “They have heard and seen lots of things on the internet, their mobile phones or from friends, and they need to know what to believe.”

At Fernwood, pupils learn to put condoms on models of erect penises. And “beer goggles” that blur the vision are used to demonstrate the effect of alcohol on people’s ability to use contraception. “Humour can be a great vehicle for learning, but it is always properly controlled,” he says.

The school also relies on theatre groups and external professionals to entertain and inform students in the classroom, adds Mr Bone, and to help fill in any gaps in teacher training on SRE.

One such group is the Christopher Winter Project (CWP), a training company that has worked extensively in Hackney, north-east London, which has seen one of the most dramatic falls in teenage pregnancy rates in the country.

A decade ago, Hackney had a teenage pregnancy rate 65 per cent above the national average, making it one of the worst five boroughs in the country. But since then it has fallen outside the top 20.

CWP’s training sessions include team-teaching SRE lessons. Trainers start by leading a lesson, then slowly withdrawing and becoming supportive observers.

Typical classes involve pupils responding to a range of fictional concerns in an agony aunt column. Older classes will have a contraceptive workshop, while younger pupils are more likely to do paired activities. No topic is too embarrassing; nothing is off-limits.

Paula Power, founder and managing director of CWP, says new teachers still need help before they feel comfortable with SRE. “A lot of teachers do not know where to start,” she says. “They feel out of their depth. They do not know how to answer questions because they are unsure about what the school policy is or where they stand within the law. If no guidelines or training is provided, they may not know how to pitch it.”

Sex may sell everything from ice-cream to cars in today’s society, but schools are still incredibly wary about how they will be perceived if they broach the subject too explicitly. Even schools that have been identified as beacons of best practice are nervous. Requests by The TES Magazine to observe SRE lessons were turned down.

“Ridiculous things have been written in the media, which can result in extended battles with parents,” Ms Power confirms. “It’s a sign of the times that schools are anxious. Most carefully negotiate with parents what is to be included in the syllabus and they do not want that hard work undone by sensationalist headlines.”

Yet this cautious attitude could inadvertently expose pupils to groups with their own agenda. In an effort to avoid bias concerning abortion, for example, many teachers would rather invite pro-life or pro-choice speakers to handle the subject.

But this can also backfire, argues Lisa Hallgarten, director of Education For Choice, an organisation that gives pro-choice talks in schools. Although there are plenty of anti-abortion groups, she explains, there are none that would actively promote abortion.

“It is a false dichotomy,” she says. “Pro-choice is not the opposite of anti-abortion.” She says she has heard of agencies misinforming pupils, falsely linking abortion to infertility or breast cancer, or likening it to a Caesarean.

Ms Hallgarten would rather do herself out of a job and ensure teachers have the capacity to deliver a balanced argument on the issue themselves. But she is not convinced most have either the ability or the desire to do so. “Provision is really patchy at the moment,” she says. “That is the only way to describe it.”

While some teachers have the confidence to give accurate, age-appropriate - and, if required, graphic - information to pupils, others are not addressing the issues at all. “Surveys are constantly telling us there is too much focus on the biological, and not enough on relationships,” she adds.

“Young people want to discuss sexual pleasure and explore their thoughts and feelings. They want to know when it is the right time to have sex and with whom. To be useful, lessons have to be more personal to their situation.”

Without a matter-of-fact discussion, pupils could be left suffering in silence, unsure if they are “normal”. If playground tittle-tattle, social- networking sites, advertising and pornography are their main sources of information, they can end up with serious distortions of reality.

It is not surprising that for a generation whose ideas about sex have been filtered through images of models and porn stars, most of them have grown up uncomfortable with their own bodies. Mrs Castor believes this is one of the reasons behind some women’s desire to have a “designer vagina”, an operation that involves altering the appearance of the vagina to make it conform to a standard aesthetic. According to research commissioned by goodsurgeonguide.co.uk earlier this year, a quarter of women want to undergo surgery on their vagina.

If adults are to have healthier perceptions of their bodies, and healthier relationships, work has to begin before such insecurities can take hold, argues Ms Emmerson. Homes and schools are the perfect forums, and if parents are unwilling to step up, schools should take the lead.

She believes pupils need to be equipped with three things: knowledge based on factual information, good communication skills and an understanding of different values and attitudes.

“It is a lifelong learning process that extends into adulthood,” she adds. “But it has to start with honesty, accuracy and a lack of complication that can sometimes be hindered by embarrassment.”

A more honest approach to sex is not just a question of busting taboos. When there is health and lives at stake, there is no time to be coy.

*Name has been changed

WHAT YOUNG PEOPLE THINK:

- 40 per cent thought the SRE they had received was either poor or very poor.

- 61 per cent of boys and 70 per cent of girls reported not having any information about personal relationships at school.

- 73 per cent felt that SRE should be taught before the age of 13.

Source: Survey of 20,000 young people aged under 18; UK Youth Parliament, 2007.

• Original headline: The bottom line