More than half a million pupils are being taught in leaderless schools because of the headteacher recruitment crisis.
A survey by the National Association of Head Teachers (NAHT) showed that more than 1,200 state schools were operating without a full-time head, and one in four heads said they would consider leaving their jobs if the "culture of excessive hours" in schools continued.
Many heads are having to take more classes because they cannot afford to take on extra staff to cover for teachers - now guaranteed 10 per cent of time off from the classroom for marking and preparation under a new contract.
Others said they were suffering from "football manager's syndrome" - forced to carry the can if targets were not met and for poor showings in school exam league tables.
Mick Brookes, general secretary of the NAHT, said at his annual conference in Harrogate yesterday: "When I first became a head 27 years ago, there were 30 applications for the post. Yet my own school, a successful and confident organisation, had to advertise twice to gain the right candidate this time last year."
Mr Brookes, who quit as head of Sherwood junior school in Nottingham to become head of the union last year, added: "Even though I left the school in very good hands in the interim, I know first hand the disruption and uncertainty that this situation can cause."
Tim Benson, head of Nelson primary school in east London, added: "The vulnerability we now have as heads is 'football manager's syndrome'. We who have committed ourselves to the most vulnerable schools perform badly in the league tables and suffer a crisis in recruitment as a result.
"It is precisely the people like myself who are actually in a deprived area who feel most threatened."
The survey revealed that one in five heads were being forced to spend between 40 and 100 extra hours in the classroom annually as a result of the new teachers' contract.
It showed that 90 per cent felt stressed or anxious part of the time while they were at school. Twenty-eight per cent said they felt stressed all the time, and two-thirds had had to take time off because of illness.
Liz Paver, head of Intake primary school in Doncaster, South Yorkshire, and a former president of the union, said: "We are as vulnerable as football managers.
"If we had clones of 30 pupils every year we would be able to keep our league table positions, but children are individuals and education and learning is about getting the best out of individuals."
The survey comes at a time of growing tensions between the NAHT, the largest headteachers' organisation, and the Government.
Mr Brookes admitted to feeling "snubbed" by ministers, who are to boycott the union's conference because of its decision to withdraw from the new teacher's contract agreement.
"Without a shadow of doubt, they're snubbing us," he said. "We have that from inside. The NAHT has more than 28,000 school leaders in membership. It is not just that they're refusing to send a minister here - they have also refused to let their civil servants attend. There are a lot of people in the Department for Education and Skills who are as worried as we are about it."
The boycott was agreed because of the NAHT's decision to withdraw from the agreement, a spokesman for the education department confirmed. Tony Blair has addressed the NAHT's annual conference twice since 1997.Reuse content