Last December Tamsyn Imison’s husband faxed her at work to say there was a message from the Prime Minister waiting for her at home. “At first I thought they wanted me to take over a failing school,” she says. “Then I thought it was a joke.”

She was wrong on both counts. The news was that Mrs Imison, headteacher of Hampstead School, a comprehensive in north London, was one of 58 people in education who had been recognised in the New Year’s Honours list. This week she went to Buckingham Palace to be made Dame Commander of the Order of the British Empire.

When he announced the list, Tony Blair said he was rewarding not only the best members of the profession but the profession itself. Dame Tamsyn, as she does not plan to be known, responded in kind, saying she accepted on behalf of all successful comprehensives. “I regard it as a representative honour. Any one of a number of heads could have been chosen,’’ she says.

But they weren’t; she was. So what makes a teacher a Dame? It is clear, on entering her office at Hampstead School, that even before the visit to Buck House Tamsyn Imison was a grande dame. A stately presence with amber and amethyst on her fingers and large silver earrings cast from seed pods of honesty, she sits on one of three huge, squashy couches, and presides over the cafetiere with a settled confidence. From the tips of her highly polished, brown lace-ups, to the top of her auburn head she radiates a good-humoured authority.

The room hints at the polymath she undoubtedly is. Tamsyn Imison, 60, has a background in both science and the arts. She read zoology at Somerville College, Oxford, before leaving prematurely to marry literary agent Michael Imison. (In those days Somerville would teach only unmarried female undergraduates.) She went instead to art college, then finished her zoology degree at London University, before embarking on a career in publishing, doing illustrations and working on scientific exhibitions. She trained as a teacher in her thirties.

Her room displays an eclectic mix of shells, crystals, stones, plants, sculptures, prints, theatre bills and children’s drawings. Unusually, it sits almost in the playground, with two sides facing out to the children. The Dame has her ear to the ground.

Although widely admired as a careful listener, she is also an enthusiastic talker. She chats engagingly about colleagues and children, about some of the idiocies of league tables, and about her commitment to comprehensive education. It is some time before I notice that I haven’t asked her any questions.

When I do, it is on the subject that she seems least keen on: Dame-dom. She slides off it almost instantly into tales of accolades she’s had from children (especially difficult ones) and their parents, and how much they have meant to her. One father, a Bosnian Muslim refugee, gave her a piece of the 16th-century Mostar bridge (notoriously destroyed by Croat soldiers in 1993) after his daughter did well in her exams; Hampstead School is renowned for work with refugee families. Another boy, source of endless angst, sent her a Christmas card saying she was the “best head”. Mrs Imison values that as a “real accolade”.

Tamsyn Imison became head at Hampstead in 1984, and 14 years in the job takes staying power, even in an improving school. “To maintain commitment over time, revisiting what went wrong after the first six- or seven-year cycle, takes stickability. It’s terribly important that some do choose to do that,’’ says one former Camden head.

The early years were difficult. In 1984, teachers were taking strike action, boycotting meetings and walking out of classes. Tamsyn Imison’s belief in “positive discipline” was resisted by some of the old guard in the school, and a group of left-wing governors were obstructive. Her staunchest supporter, Rene Branton-Saran, was ousted as chair of governors. “There were times when I felt, ‘God, I’m carrying this school on my own’,’’ she says. “But parents were wonderful, and so were children.”

Professional travails were heaped on personal tragedy. What the interviewing panel at the Inner London Education Authority had not known - because Tamsyn Imison had sworn the chief inspector to silence - was that just before she applied for the headship her son had died. (“She didn’t want to be appointed out of sympathy,’’ says Rene Branton-Saran.) The youngest of three children (he had two sisters), Thomas died suddenly at 17. He took the family dog for a walk one evening after supper and did not return. He had collapsed and died from a heart condition, so suddenly that the police initially thought he had been murdered.

Soon afterwards, a boy from Hampstead School was killed on the railway line opposite the school. “In so far as one can ever comfort parents in that situation, she did so,’’ says Rene Branton-Saran, now 76 and still a vice-chair at Hampstead and occasional lecturer at the Institute of Education.

The death of her son has contributed to Tamsyn Imison’s philosophy of life. For such a high-achiever, she places great emphasis on living for the moment, and enjoyment. “I always say to the children that I only have one rule here,’’ she says. “That’s that you’ve got to be kind to each other. I think it’s so important that you enjoy life and you are happy.’’ “Fun” heads her list of recreations in Debrett’s. She swims every morning for half an hour at the school pool, is an enthusiastic theatre-goer and beats a frequent retreat to her Suffolk cottage - often accompanied by staff from Hampstead School or other friends and colleagues.

Her sense of fun contributes to her good relationships with staff and children. “She has a large measure of the adolescent in her,’’ says one staff member. “She’s naughty.’’ This must go some way to overcoming the gulf in class and age that is apparent between Tamsyn Imison and her pupils as she moves around the school picking up stray crisp packets and Coke cans and calling big, streetwise, boys “darling” and “poppet”.

Outside her office she seems smaller, her manner with children verging on the tentative. “I’ve never known a headteacher as courteous as she is with children,’’ says a colleague. “It’s partly her natural disposition and partly her belief that they will mirror your behaviour.”

Covering a Year 7 English class, she introduces herself, and begins to teach plurals. “I’ve been told you are an outstanding group. Is that right?’’ She has no difficulty with this class as she moves around the class, giving out bonus points as she goes. “My goodness, this is superb! Excellent! Two bonus points! Can you just add an ‘r’ in church?’’ Hampstead School is in many ways a model, because it has achieved academic success without compromising its status. GCSE results are above the national average: 60 per cent A-C grades last year, and 97 per cent of children getting at least one GCSE between grades A and G. Four-fifths of the children like the place enough to stay on after GCSEs. But this rests not on a privileged catchment area; despite its name, Hampstead comprehensive is situated in unglamorous Cricklewood and primarily serves two large council estates. Nor is there any overt or covert selection. Increasingly the middle-class families from nearby West Hampstead can’t get their children in as the catchment area shrinks, now down to seven-tenths of a mile.

Hampstead School has been re-made in the image of Tamsyn Imison: the children are as important as the learning process, and staff are similarly valued - professional development is the order of the day at Hampstead. Tamsyn Imison, as ever, leads from the front. She has completed her Open University masters degree in education, is planning a doctorate, and expects staff to be equally active learners. “We don’t take on anybody who we don’t think has the potential for further development and promotion,’’ she says.

This attracts ambitious, positive staff (five former deputies have gone on to headships) but can lead to a competitive atmosphere. “At times you do feel you are working in a hothouse. There is a certain pressure to perform,’’ says one staff member.

The energy and enthusiasm that Tamsyn Imison brings to her projects, whatever they are, is both her greatest strength and the thing that other people can find difficult. Asked to describe her management style, friends and colleagues use words like “innovative” and “inspired”. But she can leave bruises. “She’s got a low tolerance of things that she regards as flannel, or getting in the way of something she wants to achieve,’’ says one colleague.

Fellow Camden heads speak with a mixture of admiration and resentment of her energetic championing of her own school. “She can drive everyone potty and will dominate almost any discussion,’’ says one. “But that’s a necessary quality in a head. If you don’t fight for your school, who will?’’ Her supportive attitude to colleagues and her personal warmth are undisputed.

Reassuringly, Tamsyn Imison’s inspiration is at least partly fuelled by perspiration. She has a ferocious appetite for work and gets by on about five hours’ sleep a night. She is active in the Secondary Heads Association, sitting on its council and chairing committees on equal opportunities and IT. Her membership of professional bodies runs to 14 lines in her CV, and ranges over special needs, science, arts, theatre, management and feminist organisations. “Courses, seminars and in-service training” occupies almost a whole page.

In some ways a natural choice for the Government (although she was nominated by Camden council), she is a popular choice among pupils, parents, staff at Hampstead School and other educationists. Her commitment to comprehensive education and to her pupils strikes a chord. “Her go-getting is not for personal aggrandisement,’’ says Rene Branton-Saran. “It’s for causes she believes in.”

Tamsyn Imison herself says she is still getting used to being a Dame. She could more easily picture herself as a knight - ”`I’d have a sword and go round brandishing it’’. Nevertheless, even as a mere Dame she will continue to ride her hobby horse for the benefit of all, and tilt at selection, under-funding and anyone who does not value learning.