It’s hard to pinpoint exactly when the sea change came. We might focus on Estelle Morris’s accession as Secretary of State in June last year, Chris Woodhead’s departure as Chief Inspector earlier the same year or the rash of headlines the previous autumn that screamed about schools being closed due to teacher shortages. Whatever date we choose, there is no doubt that the central ground of Britain’s education debate has shifted over the past two years.

For a quarter of a century after Jim Callaghan’s 1976 Ruskin lecture, Labour and Conservative policymakers had been united on the need to increase accountability and improve the performance of the teaching force. Woodhead was only the brashest exponent of an approach which has brought a succession of initiatives aimed at squeezing ever-greater productivity from the sector. Recently, however, severe teacher shortages have begun to re-order priorities.

The crisis in recruitment and retention is not a new phenomenon. Alarm bells have been ringing since the mid-1990s. Nor is Callaghan’s agenda on its way out: a Labour government which holds improvements in education as one of its main principles shows little sign of discarding the emphasis on standards. While David Blunkett, Estelle’s predecessor, staked his job on results for 11-year-olds, her reputation is likely to stand or fall on her response to teacher shortages.

Last summer, Mike Tomlinson, Woodhead’s successor, said they were the worst he had known. The last full survey, in January 2001, showed 4,980 vacancies. The number of posts unfilled or covered by supply staff was 25,000. Estelle herself warned in November that that figure was likely to rise to 40,000 by 2006, roughly 10 per cent of all teaching posts.

These statistics are probably the tip of the iceberg. Ministers are bracing themselves for a wave of retirements in the next few years since about 10,000 teachers per year have been bottled up in the system since the Government reduced early retirement in 1997. Just under half of the teaching force is over 45, so more than 160,000 are expected to go in the next 10 to 15 years.

Expert John Howson says yet more vacancies lie in the system: “If a school has a vacancy it cannot fill, it has three options: it can continue to show the vacancy, suppress it by reducing the timetable or avoid it by recruiting outside the required specialism and using non-specialists to teach the shortage subject.”

Howson says it is understandable that schools often choose to recruit outside specialisms for posts that are difficult to fill, rather than hold vacancies. This raises the possibility that real shortages in problem subjects such as maths and science may be a lot worse than figures indicate. Christine Whatford, director of education in Hammersmith and Fulham, estimates that most teachers of maths in the capital are not qualified in the subject.

Nationally, there were estimated to be 40,500 maths specialists in English and Welsh secondary schools in 1983. By 1997, that had dropped to 25,200. Now, Howson believes, there are probably fewer than 20,000. In a speech to the Social Market Foundation in November, Estelle Morris said the Government would have to recruit 40 per cent of all maths graduates even to hit her official targets.

The Government’s response to the crisis has so far been organised and had measurable success. Training salaries and golden hellos seem to have boosted applications by between 2 per cent to 50 per cent. More recently, the Government has shown a renewed sensitivity to geographical variations: pound;44 million given to schools in December was aimed at the worst affected areas, notably parts of London and the South East with very high living costs.

But the Teacher Training Agency is already hearing complaints that the allocation of teacher training places to colleges on the basis of the quality of their courses, rather than their location, is exacerbating the problem. Last year, this led to Yorkshire and Humberside, which had only 60 primary vacancies, having 273 extra places while London, with 820 vacancies, got only 198 places, despite strong anecdotal evidence that new teachers tend to stay near their training patch at the start of their careers.

Phil Turner, director of education in Newcastle, one of the country’s least-affected areas, admits he is “not on the rack” but says the city’s more deprived inner-city schools still suffer serious problems. “It is a picture replicated across the country, regardless of costs of living,” he says.

Dorset’s recruitment strategy manager Richard Davis reports big problems in recruiting to isolated rural schools with small staffs that are ill-equipped to deal with any vacancy.

“For younger teachers, they are seen to be too far away from any social activity. They can often have quite challenging pupils and there is a serious problem for the partners of teachers in two-income households getting a job.”

The age profile of the profession brings more problems for planners. In the past decade, the number of teachers in their thirties fell by about 38,000 as the huge band of 50-somethings prepares to retire. Even if the Government hits its training targets, an uneven age spread of teachers means that schools may face a worsening crisis in finding heads, deputies and senior teachers. Ministers have been looking urgently at the retention and development of experienced staff. Research also suggests that 58 per cent of secondary trainees never make it beyond three years in the job, so millions of pounds are being made available through the Standards Fund for career development.

Add to these factors the growing concerns over the retention of experienced black and Asian teachers and the shortage of men entering the profession, and it seems the shortage is actually a series of distinct crises, rather than a monolithic catastrophe.

Poor Estelle might be forgiven for wishing she had been landed with a less treacherous brief - say, Transport or Northern Ireland. But there are reasons for optimism as long as one-size-fits-all solutions are avoided. Pupil numbers in primary schools are expected to subside steadily over the next five years, and this will significantly reduce the pupil-driven demand for teachers by the end of the decade. By 2004, the same effect will be felt in secondary schools. On top of that, the drive to increase the number of young people in higher education is likely to expand the pool from which the teaching profession recruits. Perhaps most hopefully of all, the immediate future of the economy looks dismal. Teaching’s most enthusiastic recruiting sergeant has always been a dark and dingy jobs market.