Maverick headteachers are being prevented from doing the best for their pupils by the current school inspection regime, the leader of Britain's biggest headteachers' organisation will declare this weekend.
Russell Hobby, general secretary of the National Association of Head Teachers, will tell his annual conference on 3 May that inspections by education standards watchdog Ofsted must undergo a "radical reform".
In particular, he wants to see the "outstanding" grade attached to inspections abolished and replaced by a simpler pass/fail system.
Mr Hobby will say that - far from delegating power to schools - the Coalition Government had handed it to the chief inspector Sir Michael Wilshaw.
"Schools must now spend too long guessing what the inspector wants instead of thinking what the pupils need," he will add. "I have come to think that one of the most pernicious aspects of our inspection regime is the 'outstanding' grade. We have handed the definition of excellence to our regulator rather than owning it as a profession."
He will argue that excellence cannot be determined by conforming to an inspection checklist or framework, adding: "Worse than that, the acceptance of someone else's definition of outstanding creates a compliant profession. It exerts a hold over those leaders who should be most self-confident and critical.
"The outstanding grade tames mavericks ... Let me be explicit. I believe schools should be outstanding. I just don't think we should let a regulator define it.
"We have had too many reforms over the last few years but we need one more: radical reform of inspections is overdue."
Speaking last night, Mr Hobby said he would like to see more creativity "flourishing" in schools.
Kim Johnson, vice-president of the NAHT and headteacher of Bradfields Academy in Chatham, Kent, added: "It should be about being a risk-taker and being creative about what they want to do."
Sometimes, heads were too concerned to speak out because they believed Ofsted would think what they were saying was wrong.
Mr Hobby will also call on heads not to make "crazy schemes" thought up by governments work, telling them: "It is possible to make a good idea fail and, frankly, it is possible to make bad ideas succeed.
"You've proven that time and again, in rescuing the Government from its own mistakes. Perhaps you should stop doing that - it only encourages the crazy schemes when you find a way to make them work."
Last night he cited as example the introduction of free school meals for all five to seven-year-olds which, he said, was a good policy hastily introduced without thought for the consequences, i.e that many primary schools did not have kitchens. Another was the rush with which the Government introduced its exam reforms.
He foresaw - in the future - that headteachers' leaders could resort to legal action if they believed a scheme was impossible to implement. "If people don't feel an offer of consultation will make any change, we could go down the legal route," he said.
In 2013, headteachers toook exams regulator Ofqual and the exam boards to court over the previous summer's GCSE English results after the grade boundaries had been changed between the January and June sittings.
In his speech again, he will warn: "To paraphrase a famous note: there is still no money. We are merely halfway through austerity and half a million new pupils are coming our way. Expect tight budgets and make every penny count."
He will argue against encouraging schools top compete with each other - and urge co-operation instead. "There is no gain in improving one school while harming another," he will say. "There is no pride in raising results by shuffling pupils around. We must take some responsibility for each other and ensure that no school is left behind."
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