There’s something cultish about International Baccalaureate teachers, I realised as I listened to yet another waxing lyrical about how teaching the diploma had changed her life. “The movement is like a big family. That’s what’s so great. You are supported and stimulated by all these amazing colleagues from all around the world,” she said, beaming.

Debate over the baccalaureate has grown since the Tomlinson report - and so has its popularity. Only a handful of British schools offered it in the early 1970s. This year, 79 are qualified to do so, and the number is forecast to grow to 200 over the next few years. The International Baccalaureate Organisation, which licenses schools to operate the system, says it is struggling to meet demand for training. It will not be long before it becomes a consideration for job applicants: do I want to work in a school that doesn’t teach the baccalaureate? Where can I get the training so I can apply to one that does?

Some converts see the baccalaureate as a potential substitute for A-levels, able to offer a more accurate measure of academic achievement; others simply see it as a better way of delivering post-16 school education.

Sceptics, including die-hard A-level teachers, mutter darkly into their test papers about the diploma’s academic shallowness and its lack of appeal to universities. But the joyful conviction of the evangelists is unshaken.

Traditionally, the baccalaureate has been a portable qualification, the darling of the expatriate community. But these days, contrary to perception, it is no longer the preserve of blue-chip grammar schools or the private sector.

“Personally, it’s been fantastic. I’ve been a headteacher for 15 years, and it’s given me a new lease of life,” says Ian Andain, head of Broadgreen comprehensive in Liverpool. He introduced the baccalaureate to the sixth form at his school in 1992, and says he has never looked back. “My colleague heads in Liverpool thought I was mad at the time. But the quality of both teaching and learning at Broadgreen has improved tremendously.

Although the programme is for the sixth form only, the diploma has made a remarkable impression on the whole school.”

Broadgreen is, in many ways, a typical inner-city comprehensive. Many pupils are entitled to free school meals and it has a significant proportion of children with special educational needs. Yet Mr Andain says he has no difficulty attracting staff, and, indeed, there is no one teaching a subject they are not qualified to teach.

Sixty-three of his 157 sixth-form students are taking the International Baccalaureate - the rest are on vocational programmes - but all benefit.

The community voluntary service required for the baccalaureate improves the ethos of the whole school, as does the international flavour lent by children whose parents have come from overseas to work in the UK.

“We have an international sixth form. The baccalaureate has totally changed its character for the better,” says Mr Andain. He denies the perception that the diploma is too demanding for less able students, arguing that those who cannot manage the full programme can still expect to be offered a university place with a handful of certificates in individual subjects.

What most baccalaureate teachers enthuse about is the chance it offers to get away from an exam-oriented system and back into what one teacher calls “real learning”. They speak enthusiastically about the breadth of the knowledge acquired during the diploma course, which is decided by a combination of final exam and coursework.

Critics say that this breadth comes at the expense of depth. Anne Smith, who used to teach baccalaureate English at a high-achieving Kent grammar school, found the emphasis on synoptic analysis - comparing themes across world literature, for example - required both too great an intellectual maturity for many otherwise capable students, and discouraged them from really careful analysis of texts.

This argument gets short shrift from Jane (not her real name), who teaches both baccalaureate and A-level history at another Kent grammar. “That’s nonsense. The baccalaureate is turning out excellent historians who have learned to think for themselves. With A-levels you’re actually telling the kids not to bother learning certain topics on the syllabus because you know they won’t come up. You are playing the system to get them through. That’s not education.”

Another issue has been whether certain degrees - particularly medicine and the sciences - require specialisation not offered by a diploma. Jill Rutherford, adviser on the baccalaureate to Oakham school in Rutland, and director of Ibicus, a company that runs training courses for IB teachers, bats that one away.

“The only degree the baccalaureate would not combine with is Cambridge maths. There are five school subjects at its core, plus a 4,000 word essay and a course called theory of knowledge. For greater specialisation, it is easy to drop, say, one arts course and substitute an extra science.”

What about the administrative headache caused by having to teach both A-level and baccalaureate classes in schools that run a tandem system?

“It’s worth it,” Jane says, adding that the diploma has restored her faith in her job. “I was getting really jaded with A-levels. They are so reductionist. A-levels are devalued, and you feel devalued for teaching them.”

Other teachers warn that the biggest adjustment to make on switching to the International Baccalaureate is to remember that students’ workloads are much heavier. “It is a quantum difference from A-levels. You are no longer teaching one of just two or three closely related subjects. I had to stop myself from flogging my kids to death,” says Sandy, one of Jane’s colleagues.

Converts argue that the pay-off of job satisfaction and development far outweighs extra workload. The International Baccalaureate Organisation requires that a proportion of teachers at schools licensed to teach it regularly attend refresher courses. On top of the obvious attraction - a free weekend in Geneva at the organisation’s headquarters - there is also the stimulation of updating skills and knowledge and of meeting baccalaureate teachers from around the world.

For Mr Andain, administering a modest state-school budget, the advent of budget airlines has been a godsend. He admits that without help from corporate donors in Liverpool, and his local authority, he would have found it almost impossible to meet start-up costs of registration and staff training abroad. But he reckons it is money well spent. “This opportunity to meet teachers from all over the world, coming to the diploma with different perspectives, is just so stimulating. It gives us a greater depth of understanding of teaching,” he says.

Parents too, it seems, are increasingly convinced of the baccalaureate’s virtues. Myra Macdonald, a senior foreign correspondent with Reuters news agency, says there was simply no other practical choice when she moved with her daughter, Hannah, first to Paris, then to New Delhi, then back to London. But even if she had never left the country, she believes the International Baccalaureate offered her daughter something the English system could not.

“It is a window on the world. With the arrival of the global village it is more important than ever for English kids to see that there is more to life than their own little country - all the more so if they’ve never lived abroad,” she says.

Ibicus offers workshops where teachers who are new to the IB can work through all aspects of their subject with experienced teachers.

www.ibicus.org.uk