The theory - to keep good teachers in the classroom - was a good one. But the practice has been less straightforward. When, five years ago, the Government introduced a new pay scale and professional title for “advanced skills teachers”, the reaction was almost universally negative. The unions predicted “superteachers” would divide the profession and breed resentment in the staffroom.

After a slow start - by September 2001 there were just over 1,000 - the number of ASTs has risen steadily to 3,500. More than three-quarters are in secondary schools, and they are distributed unevenly around Britain. Leeds, for example, has 118, one of the highest concentrations, whereas Sheffield has just 13 and Wales none. There are 82 secondary schools with five or more and thousands of schools with none. The core subjects, perhaps unsurprisingly, are the best represented: English has 552, science 407, and maths 399. Art, craft and design have 111, PSHE has only 41.

Teachers become ASTs only after a rigorous one-day assessment by a government-appointed agency, and submitting a “portfolio”. If accepted, they can earn from pound;30,000 up to pound;53,000 (in inner London).

They - and their schools - are expected to devote 20 per cent of their timetable to sharing good practice in other schools as well as in their own.

Last October, Ofsted reported that ASTs had “significantly improved” teaching and learning in three out of four schools where they worked, and in August, the Department for Education and Skills announced continued funding for the next two years. By autumn 2005 the aim is to have 10,000 ASTs, representing five per cent of the workforce.

Even the unions have softened their position. The National Union of Teachers, which effectively halted AST recruitment in 2001 when it challenged threshold payments in the courts, has “lowered its radar”, says its head of education, John Bangs. While he disagrees with the “crazy pay structure” and the way it was introduced, he now admits “there are some ASTs doing a cracking job”.

Gwen Evans, deputy general secretary of the Association of Teachers and Lecturers, says:“We have heard about more good work than bad, so on balance our attitude now is positive. Where an AST has advanced the school, that’s something to celebrate.” And Chris Keates, deputy general secretary of the National Union of Schoolmasters Union of Women Teachers, says: “We don’t get a lot of concerns expressed by members; people should get the highest rewards for remaining in the classroom. We have a rather ambivalent attitude.”

ASTs have become a part of the educational landscape. But who are they, what do they do, and can they live up to the superteacher tag?

The walls of Gill Greany’s textiles studio are lined with colourful creations. “Works of art in themselves,” she says, casting an eye over the delicately constructed collages, needlework and sketches of students past and present.

Ms Greany, 52, is fortunate to teach at Beauchamps college in Oadby, Leicester, which has not only one of the highest numbers of ASTs in Britain - 11 - but also, with nearly 1,000 students, the UK’s biggest sixth form.

It means a minority subject such as textiles can be run as a full A-level in a thriving department that also offers photography, art and graphics. Ms Greany’s students are lucky to have her as a teacher: 32 per cent of them will achieve A* or A grades, and around half will go on to study textiles or a related subject such as fashion. Ms Greany keeps a book of former pupils going back 10 years, with occupations; it’s full of teachers, boutique owners and fashion designers, many of whom she’s still in touch with.

“I like building that relationship with students; you get so much back from it,” she says. Ms Greany, an AST for three years, became an Ofsted inspector in 2002, but finds it difficult to turn off her teaching instincts when observing other people’s lessons. “I get so twitchy - I want to dive in and start teaching them myself,” she says. “When I come back I’m always in such a good mood the kids all say, ‘Where were you last week?On holiday?’ But I say, ‘I’m just glad to be back teaching you lot’.”

Today’s students are making a Panamanian blouse with layered, elaborately stitched motifs. Ms Greany uses fabrics from around the world to expose students to unfamiliar techniques; her favourites are Middle Eastern tribal rugs. Closer to home, Leicester’s hosiery industry has all but gone, although salvaged hand looms serve as working models in her classroom.

Ms Greany’s own love affair with fabric began with her needlework teacher at Tonbridge grammar school for girls, Mrs Childs. “She was wonderful. She used to take us to the theatre and the ballet, and she was a bit of a rebel; she had us making baby doll nighties.”

As an AST, Ms Greany has mentored trainee teachers and drawn up course guides and revision materials. She has also been involved in a curriculum mapping project to ease the transfer of pupils to Beauchamps, a 14-19 school. Being an AST has given her the “luxury” of time to do these things, but she says nothing compares to a good lesson. “Textiles is my passion.

One of the most important things you can do as a teacher is to be enthusiastic. If you have a good lesson you walk away feeling happy. And that’s a great feeling.”

Deoxyribose nucleic acid always was a bit of a mouthful, and it can be as difficult as it sounds for a class of 15-year-olds to get their heads around. But Jean Cade, 57, has a novel method of making the fundamental building blocks of life on earth a little simpler.

“Has everybody remembered to wash their hands?” she asks, as she moves around the class, dishing out liquorice strips, jelly sweets and cocktail sticks. This morning’s Year 10 class has the task of constructing the DNA double helix out of confectionery.

Biology and its teaching have changed a lot since Ms Cade started in the classroom in 1968, just 14 years after Watson and Crick discovered DNA.

“What we know now is way beyond what we knew then,” she says. “The stuff I am teaching was not in my degree. It has been a huge challenge keeping up to date.”

Expectations have changed too - “the demands of the job are far greater, accountability is far greater and what the kids are expected to do is far greater” - but her love of the subject has remained constant. “I don’t see how anybody can fail to be fascinated by biology - it’s life.”

Her own biology teacher was “a bit off the wall; when his wife had a baby he brought the placenta into school in a bucket”. When Ms Cade began her career fresh from university, “in at the deep end” at a Hampshire secondary, she was lucky to have a “brilliant” head of department who trained her on the job. As a child, Ms Cade had moved around with her father’s RAF postings, and her rural childhood homes and mother’s interest in nature left her with a love of the countryside.

But not all her students at the Deacon’s school in Peterborough - where she has taught for 19 years - have had the same upbringing. She remembers a trip when one of them couldn’t identify a common flower. “I was incredulous that a kid could grow up not knowing what a buttercup was.” Field trips remain one of the highlights of her job.

Ms Cade took on the role of AST five years ago and threw herself into a variety of roles, teaching everywhere from primary schools to universities and overseeing the graduate teacher programme at Deacon’s, which is currently home to 20 trainees. Last year, she says, the extra demands on her time started to affect her classroom work. “I came out of a lesson one day and thought, ‘That was dull’.” So she eased back and made more time for her first love - teaching. “I thought, ‘If I carry on like that I’m going to get bored’. And if there’s one thing teaching shouldn’t be it is boring.”

There’s no such thing as a typical day for 37-year-old Simon Elledge, the primary AST at Wandsworth City learning centre in London. Typical weeks are pretty infrequent too. And a typical term? An ICT specialist, Mr Elledge takes his truckload of digital equipment into a school for several weeks at a time, seeing a music or video project through from concept to end product - usually a CD or DVD that the children can take home.

He reckons that by the end of the year he’ll have taught around 1,200 children. On top of that are the 170 teacher trainees who rely on him for their IT training through his links with five institutions. “They are all potentially taking something into the classroom that I have given them, and that’s a buzz.”

Until 18 months ago Mr Elledge was ICT co-ordinator at a primary school in Wandsworth and, with a family to support, he was taking on any extra work to inch up the pay spine. “It was getting ridiculous; I was jack of all trades,” he says. A secondment at the City learning centre led to a full-time post, and in April this year he became an AST. “It has lifted the burden,” he adds. “I work as hard as I did but I only have to focus on one thing. I have gone from five jobs to one, and my wife says I am a different person.”

Mr Elledge qualified as a teacher seven years ago, at 30, following a period working at a school for autistic children. “I was inspired by the head there. The patience and understanding he had - I had never seen anything like it.” Leaving college with student debt and a child on the way was difficult, but the financial rewards of deputy headship didn’t appeal.

“I didn’t want the responsibility and the admin pulling me away from the classroom. Being a head is like being an accountant and a lawyer these days. I’m not interested in that; I’m interested in children’s education.”

This year he has overseen video productions of Chaucer, the news and weather Tudor-style, and a streetwise safety project for pupils starting secondary school. In a world where children can watch dozens of television channels, store hundreds of songs on a personal stereo the size of their hand, play video games with cinema quality graphics, and “where even the sign at the bus stop moves”, they need something more stimulating at school than a blackboard.

One project sums up the transforming effects of technology. An after-school music scheme at eight primary schools this year produced a 23-track CD and accompanying pop videos. “Listen,” he says, as an anti-war rap booms out from his 16-track mixing desk. “Can you believe that was written, performed and produced by eight-year-olds?”

His enthusiasm for the job is, he hopes, infectious. “I love teaching and I love teacher training,” he beams.

Roy Watson-Davis, a history teacher at Blackfen school for girls in the London borough of Bexley, isn’t happy with his title. “Advanced skills teacher is such an arrogant title,” he says. But then Mr Watson-Davis, 40, has had a few unusual job titles in his time.

After graduating from university, he worked as a supermarket stock controller, insurance salesman, manager of a chain of charity shops and milkman. “The kids find that hilarious,” he says. “I must have been the world’s most over-qualified milkman.”

He came into teaching late, at 28. “I always wanted to teach but never had the opportunity. When I was training, my assessor described my style of teaching as ‘seat of the pants’: it always looked as if I was going to have a disaster but I never did. I have always been able to pick up a lesson and run with it.”

He has always loved history, and enjoys visiting old buildings to try to get a sense of time and place. His favourite historical figure is Charles I. “I always go for the heroic failures.”

Classroom practice is Mr Watson-Davis’s strength (his teaching tips have featured in The TES). He likes to play around with conventional set-ups by putting his desk and the blackboard at opposite ends of the classroom and displaying the work of several year groups together. He marks books after every class to give instant feedback on how much of a lesson has been understood and as a way of improving discipline. If pupils know their progress is being checked, they will, he reasons, make more effort.

Since becoming an AST two years ago, he has been using homegrown hints such as these in his work with a special school in special measures and with NQTs and trainee teachers. Support from colleagues is essential, he says.

“The outreach element takes a valued teacher out of school. Last week the head released me for an extra day because the school in special measures I support had the inspectors in. That’s how it should work.

“You don’t become an AST without the help and support of all your colleagues. The school is a team and the staffroom is a team; different people play different roles. There are no gripes about me being an AST.”

Mr Watson-Davis has been at Blackfen - a secondary modern girls’ school with a mixed sixth form, “a great school with blinding pupils” - since he qualified in 1992. Being an AST has been a good move because, he says: “I would make a lousy head of department.”

Melinda Hale had wanted to be a teacher since she was a child. “It sounds sad, but when I was five I used to arrange my dolls in a circle and draw up a register,” says Ms Hale, 32, a maths teacher at Brislington school, Bristol.

She accumulated some early work experience when her own maths teacher enlisted her help in tutoring the girls in the class. But then a teacher, of all people, nearly thwarted her ambition. “He said, ‘You are too quiet and too shy to be a teacher’ and I went off the idea.”

Studying for a maths degree, she was set on a career as an accountant when a friend persuaded her to apply for a PGCE. Accountancy’s loss has been teaching’s gain as, in 11 years at Brislington school, Ms Hale has progressed from head of year to head of department to AST. This year she was named secondary teacher of the year for the West Country in the Teaching Awards. Her career trajectory has been unerringly upwards but she remains refreshingly down-to-earth.

“I was anxious about being held up as a great teacher; we all have bad days. I didn’t want to be thought of as someone who was doing this better than anybody else.” Even when her head encouraged her to apply for AST status last year - the first person in the school to do so - she was unsure. “It took me a long time to make the decision to do it.”

Her “Ofsted for one” was less harrowing than it sounds. “Everybody said so many nice things about me, it was wonderful,” she recalls.

In the classroom, she is direct and confident. She rattles off standard index form equations, then plunges into trigonometry revision on the whiteboard and laptop that were her prizes in the Teaching Awards. This Year 11 set doesn’t have a bad word to say about her either. “Some teachers expect you to respect them straight away,” says Maria. “But she doesn’t get lairy and shout at you.” When Ms Hale has difficulty projecting a diagram on to the board, a student helps her sort it out. She’s keen to encourage pupils’ suggestions on teaching styles; last week they even videoed a class to see if there was room for improvement. “It’s good talking to children about teaching and learning,” she says. “They often have a far better idea of what works for them and what doesn’t.”

She has only ever taught at Brislington, so being an AST has broadened her experience without her having to leave a school she is so obviously attached to. One of the schools she visits had no head of maths, so she has supported and sometimes led the department. “I’m learning a lot about teaching just from watching other people and seeing how things are different in other schools.” She is also working on programmes for gifted and talented children and teaches thinking skills to staff and pupils.

Those who thought Ms Hale couldn’t do it were wrong. “Teaching and responsibility have made me more confident,” she says. “You have to find where your own personality fits with being a teacher.” Meanwhile, she’s dealing with the next generation of doubters.

“My biggest bugbear is when you hear kids say, ‘I can’t do maths’. You never hear them say ‘I can’t do history’ or ‘I can’t do geography’. I always say to them ‘yes, you can’.”

For information and application forms go to: www.teachernet.gov.ukast