Introducing radical free market reforms to state education does not lead to innovation in the classroom, a major international study has found.
It is government intervention, rather than market forces, that has often led to changes in teaching practice, according to the report by the Organisation for Economic Co-operation and Development.
Governments turn to competition between schools, greater operational freedom and decentralised school governance as a way to increase innovation and escape "deadening uniformity", the report said.
The idea is that "schools will respond to the threat of losing students and funding by innovating or otherwise improving their effectiveness," it said.
But examining developments in 20 countries, including England and Wales, the study found no "direct causal relationship" between market-driven choice and competition and innovation in the classroom.
While reforms can promote innovation in school organisation, they are "somewhat limited" in encouraging new practice by teachers.
In England, reforms to school governance, such as those introduced with City Technology College, were achieved by government intervention, not a free market, the report says.
The study presents some conflicting evidence, with the US charter school movement helping to spread different teaching practices. But researchers in some countries, including the UK, also point to freedoms leading to a "back to basics" approach.
John Bangs, head of education at the NUT, said the findings cast doubt on the academies programme and on Tory plans to open hundreds of "free schools" operated by different providers.
The Conservatives have said they plan to divert #163;4.5 billion intended for Building Schools for the Future to fund the schools, which would be operated by a range of new providers. The plan would result in more than 220,000 new places to respond to parental demand.
Mr Bangs said: "It is crystal clear that the market stimulating innovation is blown out of the water by this study.
"Structures of whatever form are not where it's at. It's what happens in the classroom and if teachers feel free, confident and well-informed enough to try out new things without fear of being hammered."
The Conservatives' education policy document says that schools that innovate and improve outcomes should be applauded. But a spokesman denied that the free school policy was designed to create innovation.
"Only someone who fundamentally misunderstood this report could think it criticised Conservative policy," he said. "It is about whether free market reforms lead to entirely new, never been tried classroom practices, not whether they raise standards.
"We have always argued that we think that a genuine choice system would lead to more tried and tested teaching methods because that is more popular with parents.
"There is a wealth of international evidence showing that a genuine choice system leads to higher standards."
The OECD report follows a study published by Stanford University in the summer which revealed that a third of US charter schools, which have significant freedoms, provide a worse education than standard state schools.
Just 17 per cent of charter schools deliver better education, while almost half made little or no difference, academics found.