It has brought the rise of a new phenomenon - locum headteachers - sent in by agencies to run schools that cannot find heads.
Headteachers, who will today submit a pay claim for an average rise of 17 per cent, say that uncompetitive pay and mounting pressure on heads to raise standards are putting off prospective candidates.
David Hart, general secretary of the National Association of Head Teachers, said: "The current instability surrounding thousands of schools which have acting or locum heads is bad for children's education.
"Acting heads do their best, but there is no substitute for a permanent head who can conduct long-term planning and improve standards over time."
In London, agencies report that they cannot find enough "locum" heads to fill all the vacant posts.
Locums are often headteachers who retired when the Government tightened the early retirement rules last year.
Mr Hart suggested that such appointments might be of dubious legality, as they were not covered by the teachers' pay and conditions agreement.
A survey conducted by Education Data Surveys for his association found that nearly two-thirds of primary headships in London are re-advertised, often because no suitable candidate comes forward. For the Home Counties, the figure is a quarter and for elsewhere 22 per cent.
Acting heads are appointed for a term or more while the search continues.
Only 17 per cent of small primary schools receive ten applications or more, compared with 30 per cent last year and 50 per cent three years ago, the survey shows. Only a third of large primary schools received ten or more applications.
Mr Hart said the level of primary school heads' salaries had reached "scandalous proportions" compared with those for people with similar responsibilities in other jobs. From next April, the head of a small primary school earns pounds 29,355 and that of a large one pounds 36,270. Secondary-school heads earn between pounds 42,603 and pounds 55,557.
An average pay rise of 17 per cent would make up the difference between heads' salaries and those for comparable jobs.
Mr Hart added: "Since headship is going to determine whether the Government hits or misses its targets by the next election, it would be well-advised to pay attention to these problems.
"If you can't get enough people of the right quality into the most responsible position in education, you have to do something about it." There is no prospect of an immediate end to the crisis.
Mr Hart said that last year just over 4,000 candidates came forward for the Government's new headship qualification, against an annual target of 5,500.
A Department for Education spokesman said: "The Government will be taking action on this. We will shortly be submitting evidence to the Teachers' Pay Review Body which is expected to cover some of these issues. The Green Paper later in the autumn will be addressing these matters in a fundamental way."Reuse content