Talented head teachers are risking "career suicide" by taking on jobs at the most challenging schools, a union leader warned on Saturday.
Association of School and College Leaders (ASCL) general secretary Brian Lightman claimed unrealistic pressure to raise standards quickly has seen many good heads lose their jobs while others fear for their futures.
He warned that head teachers are not "commodities" that can be thrown away and suggested there is already a lack of people willing to lead the most challenging schools.
At the ASCL annual conference Mr Lightman said that while head teachers who take up the challenge at underperforming schools have great motivation to turn them around they often do not get the support they need.
"I'm very worried that for many people it's becoming career suicide and that would be distressing for our schools," he said afterwards.
Education Secretary Michael Gove admitted that when head teachers are making big changes they need to have time to do so.