Some scientists and engineers - especially those who have been laid off or taken early retirement - find it hard to work in social environments after years in the laboratory, according to Colin Broomfield, a retired head and Secondary Heads Association executive member. The best mature recruits were in their 30s and had made conscious decisions to change to teaching.

He admitted that desperation drove him to appoint some mature entrants on short-term contracts. “I knew they could cope for two terms before things got too obviously out of hand and parents came to say these people could not keep control,” Mr Broomfield told the London conference organised by the Society for Research in Higher Education.

Professor Alan Smithers, of Liverpool University, told delegates that the most recent government figures - from 1996 until now - showed that six months after gaining a PGCE, 28 per cent of under-25s were unemployed compared to 58 per cent of over-45s - and 100 per cent of over-55s.

He said the figures, based on his research, could not be blamed simply on older teachers’ higher salaries. Schools worked on age-related hierarchies - older pupils took greater responsibilities and older teachers were expected to have higher status. Mature newcomers challenged that hierarchy and were less easy to mould than young ones.

“This is a major problem for the Government,” he said. “The whole thrust of its Green Paper is about bringing older people into teaching.”

The conference heard renewed calls from training providers and from Sir Clive Booth, chairman of the Teacher Training Agency, for bursaries or salaries to be paid to teaching students, to help courses compete for graduates.

In other professions they would start earning immediately with training thrown in. “Our students still have to support themselves without getting paid and their undergraduate debts go up,” Sir Clive said.