Pupils at a free school which was closed down amid concerns about standards had been "taught nothing", a union leader is claiming.
According to Russell Hobby, general secretary of the National Association of Head Teachers (NAHT), schools which had taken in students after the free school was shut had reported that some youngsters were a term behind where they should have been in their education.
It is understood the free school is Discovery New School, in West Sussex, which was one of the first of the Government's flagship free schools to open in 2011.
It was shut last month after serious concerns were raised about education standards - the first time one of these new schools was closed.
In his speech to the NAHT's annual conference in Birmingham, Mr Hobby is expected to express concerns about the pace of education reforms, arguing they have been rushed through in order to show results before the next election.
"The dangers of poorly thought through policy, rushed in to be able to claim a result, are also exemplified in the shaky results delivered by the earliest free schools.
"Some free schools are performing highly and, to be fair, few schools could have lived up to the hype attached to them, but some people were given schools to run who should not have been allowed near them.
"I have spoken to schools who have taken in children after the collapse of one free school. They reported for one group that after one term of education they were precisely one term behind where they should have been. They had been taught nothing."
Mr Hobby claimed that the approach to reform is "dedicated to cramming in enough changes to show tangible results before the next election."
He adds that the upcoming autumn term will see the introduction of a new curriculum and free school meals for infants, as well as changes to assessment and teachers' pay.
"Is it a coincidence that this is the last chance to get big changes through before the next election?" Mr Hobby said.
"No, but is that a good enough reason?"
Free schools - state schools that are free from local council control with power over areas such as the curriculum - are one of the Government's key education policies.
Three-quarters of the first 24 free schools to open to pupils - those that opened in autumn 2011, have been declared good or better by Ofsted.
A Department for Education spokeswoman said: "Unfortunately the standard of education at Discovery New School was simply not good enough. That is why we took action to close the school just seven months after its 'inadequate' rating.
"We will not tolerate under-performance in any school and, as we have demonstrated, we will take swift and decisive action if children are not getting the education they deserve."
She added that there are more than 170 free schools open across the country and the "vast majority" are performing well.