Heads who take on jobs running tough disadvantaged schools risk "committing career suicide", a teachers' leader has warned.
Ofsted was wielding a "Sword of Damocles" over "any senior leaders foolish enough to think that they will be sufficient to undertake the tricky work of turning round schools with seriously entrenched problems," Mary Bousted, general secretary of the Association of Teachers and Lecturers, told her union's annual conference in Manchester.
The education standards watchdog often delivered a verdict on schools of high challenge or high deprivation soon after the new head arrived, with the result the school got a poor rating and was forced into becoming an academy.
Dr Bousted described Ofsted as "an agency designed to inspire fear and loathing in teachers and school leaders".
In that, she added, it had been successful.
However, she argued, "the game was up" for Ofsted as it faced growing criticism, including from the right-of-centre think-tank Policy Exchange, over the standard of school inspections. "We know that, frankly, it's a lottery which depends on which Ofsted inspection team turns up - one that has a clue, or one that is clueless," she said.
In a letter to the Association of School and College Leaders, which represents secondary school heads, chief schools inspector Sir Michael Wilshaw has acknowledged that newly appointed heads need time to turn a school round, and that Ofsted should take account of the length of tenure of the head when arranging inspections.