Rising numbers of headteachers are being treated like football managers - facing the sack if they fail to deliver good results, a union claimed on Friday.
The Association of School and College Leaders (ASCL) said that the numbers of members losing their jobs for reasons other than misconduct and ill health has trebled in the last five years.
The union condemned a "football manager mentality" which sees school leaders sacked due to pressure from local councils, academy chains and central government who have unrealistic expectations about how quickly they can raise results or turn a failing school around.
Speaking as the ASCL meets for its annual conference in Birmingham, general secretary Brian Lightman said: "We are not talking about 'incompetent' heads or those fired for misconduct.
"These are overwhelmingly good school leaders who find themselves in difficult schools facing near impossible demands and timescales for improvement.
"It is perfectly possible to turn around underperforming schools but this does not happen overnight and too often the powers-that-be have unrealistic expectations about what can be achieved in a short space of time."
He claimed that in one case, a head had been judged as outstanding by Ofsted at her last inspection, but when she moved to a new academy which then was placed in special measures she was told she had to leave.
"The football manager mentality is yet another factor discouraging good people from taking on headships, especially in schools in challenging circumstances," Lightman said.
"The difference between football clubs and schools is that school leaders don't get massive contracts and huge payouts, and once they lose their job, too often their career in schools is over."
The ASCL said its figures show that 272 members lost their jobs in the last year for reasons other than misconduct and ill health.
This is up from 163 in 2010 and 93 in 2007.
In 2011, an additional 80 ASCL members contacted the union about leaving teaching because they felt demoralised. Of these, 32 left their jobs.