Like so many others, the girl had come to London in search of fame and fortune. She was 16 and living in a hostel, waiting to be discovered. To give her career a push, she arranged to meet Janet Edwards, a singing teacher who had previously taught Leona Lewis, the biggest star to have come out of The X Factor so far.

After watching the girl perform - a performance which could be described as adequate at best, Ms Edwards asked about her aspirations.

“So,” the teacher began. “What do you really, really want to do?” It was not the first time she had put this question to a pupil. Usually the answer came back instantly: I want to have a number one record; I want to be a star; I want to be famous. This time, however, there was a pause. “Oh my God,” the girl said, eyes sparkling. “I want to be a dietitian.”

“Her face completely lit up,” Ms Edwards recalls. She advised the girl to keep up her singing and dancing, but find out what A-levels she needed to take. The girl now has a burgeoning dietitian’s practice.

She is not the only one to have been seduced into thinking she could be a star. A diet of reality TV - X Factor, Britain’s Got Talent, Big Brother - has convinced a generation of young people that instant fame can be theirs for the taking. And teachers are seeing the consequences in their classrooms.

“Those sorts of programmes only show the glittering lights,” says Hilary Meyer, head of music, performing arts and drama at Coloma Convent Girls’ School in Croydon, south London. “They don’t show the difficulties that you encounter if you decide to go into a career like that.

“We get a lot of pupils doing the performing arts course I teach. But it’s often much harder work than they anticipate. In any GCSE, you are going to have written work. A lot of them are very good at performing, but absolutely hopeless at handing in their portfolios or meeting any deadlines.”

The image of reality TV as a shortcut to success can be traced back to the first series of Big Brother in 2000, which made instant stars of some of its contestants. Instead of years of graft and toil, they found easy fame.

As the programme’s final series drew to a close this summer, overshadowed by the more tangible rewards apparently available on Britain’s Got Talent and The X Factor, the fact that most of the house’s briefly-famous residents have now fallen back to obscurity has done nothing to diminish this view.

“Particularly since X Factor, I see an awful lot of people who buy into the idea that success can be instant,” says Ms Edwards. “But they’re light-years away from where you need to be.

“Singing can be a wonderfully fulfilling hobby. But people are getting diverted away from other things they could be really passionate about. You need to be careful not to confuse passion with a red carpet or platinum sales.”

Times have changed since the days when children had the mantra from 1980s TV series Fame imprinted in their minds: “You want fame?,” asked dance teacher Lydia Grant at the beginning of every episode. “Well, fame costs. And right now is when you start paying - in sweat.”

Today, fame seems to cost nothing more than the price of a bus ticket to the nearest reality TV audition. A decade of exposure to the instant celebrity world of reality TV has had a profound impact on the adolescent understanding of success.

For today’s pupils, instant success is part of the lexicon of growing up: any career can be theirs, as long as they want it enough.

“It’s particularly an issue with boys,” says Geoff Brookes, deputy head of Cefn Hengoed secondary in Swansea. He regularly witnesses 15 and 16-year- old pupils pretending to be on X Factor: a childlike game of make-believe seeping into adult life.

“What they’re talking about is the fame before the effort, before the work,” he says. “They think you become instantly famous, and then things happen to you. Most people who are successful do a lot of groundwork before appearing on TV. But pupils think it’s enough to turn up on television and shout a lot.”

Reality TV, Mr Brookes believes, has bequeathed his pupils an obliviousness to the hierarchy of hard work and ability. There is little sign that they have grasped the concept that some people are better suited to some jobs than to others; instead, pupils insist that wanting to do a particular job is qualification enough. “There is a sense that it’s what I want, therefore I should have it,” he says. “They have a sense of entitlement: I want to be a doctor, so I can be a doctor. If I can’t, it’s because my teachers are standing in the way.”

One of his pupils, for example, had severe learning difficulties. But she and her parents complained that, by refusing to let her sit higher-tier GCSEs, Mr Brookes was barring her way to a successful medical career. Similarly, this year he taught a lower-set GCSE class, attended mostly by pupils who had previously been excluded for truancy. They had not been entered for higher-tier papers because of their extended absences, but the pupils insisted that it was their right to sit the exams nonetheless.

I had a big argument with them,” Mr Brookes says. “They think that, just by turning up and doing the higher-tier paper, they’ll get an A. Teachers have professional knowledge and understanding. But pupils are dismissive of their teachers; they don’t want to listen to professional advice. They just say, `I want to do this’. And if you want, therefore you shall have.”

Lorraine Day has witnessed this assumption that life is an audition and the only qualification necessary is a willingness to turn up and wait your turn to appear before the judges. Ms Day, a personal adviser with Youth Connexions, works with schools in Hertfordshire. She says it is excluded teenagers in pupil referral units who are more likely to see the world as an extension of their TV screen.

Some people, for example, believe that a casual interest in music is enough to win them a place at music college. “Where do you sing?” the interviewers ask. “Oh, in my bedroom,” they reply. “In front of my mates.”

But they are competing against pupils who have cultivated a serious interest in the subject for years. Other applicants for that same college place will have been taking part in singing competitions since primary school, performing all over the country since their early teens.

“They discover an interest at age 15 and then think they can walk into a career in it,” Ms Day says. “You talk to them about work experience and volunteering and they don’t quite get it. They don’t see that they have to work harder and longer to make progress. They say, `Why should I do it if I don’t get paid? Why should I give my time for free?”

She has encountered several pupils who aspired to be youth workers, but balked at the idea of volunteering at their local youth centre. Another girl professed an interest in training as an air-hostess: she wanted glamour, foreign travel, free holidays. Ms Day spoke to her about jet lag, shift work and the reality of being a glorified waitress, but the girl was oblivious. “There are a lot of drop-outs,” Ms Day says. “They start a course, and then drop out in October. They just say, `God, this is too much like hard work.’”

The glamorising effect of TV is nothing new. But reality programmes allow TV glamour to masquerade as real-life: a well-lit, neatly edited version of the life everyone would like to live.

In 2007, two Belgian academics published a study in which they showed that the number of school-leavers enrolling on midwifery courses rose by 16.4 per cent after a docu-soap about midwives was screened on local television. Similarly, when the BBC docu-soap Vets in Practice was shown on a Flemish channel, interest in veterinary science courses increased by 15 per cent. Fictional programmes about hospital life have not had the same effect.

Were this effect not potentially destructive, it would border on comic. In Hertfordshire, careers advisers experienced a sudden interest in careers in mountain rescue after a TV programme was screened about the profession. Advisers had to explain to the teenagers that it would mean having to relocate: there are no mountains in Hertfordshire.

“Reality programmes impose a shape on life, and that’s very seductive,” says Mr Brookes. “There’s a belief that your own life has a shape, and that shape is determined by you. Depth of knowledge isn’t required: you can just blag your way through things.”

Inevitably, it is those teenagers who are most at risk of dropping out who buy into the mythology of instant success. High-achieving pupils who want to emulate Diversity, the street-dancing winners of Britain’s Got Talent, are happy to enrol in a dance class. But this is a hobby: they have no intention of giving up school to seek fame and fortune.

Excluded teenagers, by contrast, have more to gain - or less to lose - by chasing mirages. Jade Goody becomes a positive role-model, an instant celebrity whose lack of education paid dividends.

But Ms Day says she was the exception. “They (excluded teens) are living chaotic lives,” she says. “They do drugs, have difficulties with their parents. So TV is a huge influence. They don’t necessarily want to be actors, but they want the possessions: flash cars, nice holidays. They don’t think about how they are going to get there. Even if they do have a bit of talent, they haven’t nurtured it. They just want the material possessions.”

Job interviews, these pupils imagine, will be like the panel auditions on The X Factor or Britain’s Got Talent, with employers desperately seeking an enthusiastic rough diamond. They latch onto cases such as Kieran Gaffney, the 12-year-old drummer who failed to make it through to the semi-finals of Britain’s Got Talent.

Simon Cowell advised the boy to go away and practise, then try again next year. Kieran took his advice and the following year was voted into the final.

Teenage viewers, too, are hoping for a “Cowell ex machina” to give them their big break. “The kids are looking for someone to see a spark, take a gamble,” says Ms Day. “They’re waiting for an adult to stick their neck out, give them a chance. But that very rarely happens these days.”

“They think they will just be able to turn up and make money - quite a lot of it - without much effort,” agrees Sue Kirkham, of the Association of School and College Leaders. She regularly hears reports from her members on the impact of reality TV on teenagers’ ambitions.

“As soon as they do work experience or have a part-time job, they are shocked that things such as punctuality and hard work are so important.”

But, she argues, there has been a positive shift in the way talent show judges talk to their contestants. Where once they spoke only of success and record contracts, now they praise hard work and determination.

Andrew Lloyd Webber’s annually reprised BBC1 search for a West End star regularly highlights contestants’ years of auditioning and chorus-line experience. And the highly choreographed dance troupes of Britain’s Got Talent are a counterpoint to the “Hello, I think I can sing” approach of The X Factor.

“Young people are becoming more aware that a lot of training and time goes into this,” Ms Kirkham says. “Simon Cowell now says he likes contestants who get on with things and put in the work and don’t complain. He is making it apparent that people can’t just walk in, sing a song and get somewhere.”

Successful popstars are often like icebergs: no one sees the nine-tenths of ballast beneath the reality-TV surface. Leona Lewis, for example, was taught singing by Janet Edwards from the age of eight. She attended lessons twice a week, working through a repertoire that included classical, blues and pop music. And in between she would be learning, rehearsing, practising. “As far as the public is concerned, it’s preferable to believe she came from nowhere,” Ms Edwards says. “The X Factor is a money-making machine, but it’s actually smoke and mirrors. It trades in illusions. What it doesn’t give is a sense of the teaching that creates real depth and substance.”

That lesson is left to the performing arts teachers. Hilary Meyer and her pupils regularly watch professional companies at work, examining the hours of rehearsal behind any performance.

But Ms Kirkham argues that such lessons should not be limited to the music or drama classroom. She would like to see personal, social and health education teachers examine the gritty reality behind the TV veneer. “Young people are going to read about these people and follow what they do,” she says. “This would be a good way to show how you cope with adult life.”

As a potential lesson, she cites 2008 X Factor winner Alexandra Burke, who first auditioned for the series in 2005. She was rejected and spent three years taking intensive singing lessons, before returning to win. “She showed you can really achieve a lot if you’re prepared to work hard,” says Ms Kirkham.

But such examples should be used sparingly, cautions Lorraine Day. Entrepreneur Richard Branson is often cited as an example of the way a child with dyslexia can overcome poor academic performance and succeed. “On the one hand, qualifications aren’t everything,” she says. “But I do wonder if it gives them a slightly distorted outlook. These people are a very small minority.”

Successful celebrities must also be able to manage their own careers. “People who become celebrities also have to be business people,” Ms Kirkham says. “I know they have agents and accountants to represent them, but a lot lose all their money because the people around them are unscrupulous and they don’t have a business head on their shoulders.”

Not all search-for-a-superstar winners go on to have successful careers. As well as Leona Lewis and Alexandra Burke, The X Factor also counts Steve Brookstein and Leon Jackson among its winners, both of whom were subsequently dropped by their record labels. Instant stardom does not always translate into longer-lasting fame.

“There is a real danger that we are celebrating instant things,” says Geoff Brookes. “But all we are doing is fostering a sense of superficiality.

“Perhaps people are instinctively lazy. But there are still lots of kids who go on to do medicine when they leave school, knowing full well that it will take five or six years. It’s a bit like the difference between fast food and slow cooking. Which is more satisfying in the end?”

But as long as reality TV peddles the dream, there is not likely to be any shortage of teenagers happy to put their audition ahead of their homework.