Labour leader Jeremy Corbyn says that the plan to force all England’s schools to become academies was a step towards “asset stripping” and privatisation.
He told the National Union of Teachers conference that he backed teachers’ opposition to “forced” academies.
Mr Corbyn, the first Labour leader to address the NUT, received a standing ovation from delegates in Brighton.
Schools minister Nick Gibb said it showed the Labour party had “chosen to retreat into the fringes”.
Mr Corbyn’s speech at the NUT’s annual meeting comes more than a decade after senior Labour figures stopped attending.
Estelle Morris was the last Labour education secretary to address the conference, in 2002, where she was heckled and slow handclapped.
But Mr Corbyn received a standing ovation even before he had spoken – and his speech, condemning the government’s plans for an all-academy school system, was received with loud applause.
The Labour leader accused the Conservatives of excluding parents from how their children’s schools are run – and argued that there was no evidence that academy status improved school standards.
‘Shut parents out’
“Let’s be clear, this is an ideological attack on teachers and on local and parental accountability,” Mr Corbyn told the conference.
“It was nowhere in the Tory manifesto, it’s something that’s just been dreamt up at the last minute and stuck into the Budget.
“I want schools accountable to their parents and their communities – not as a process of asset-stripping our facilities to be handed over to somebody else.
“There is not a shred of evidence that academies improve standards.”
The Labour leader said the government’s policy was a step towards the privatisation of the state school system.
And he argued that it failed to address problems facing schools such as tackling teacher shortages or dealing with the consequences of child poverty.
Mr Corbyn’s criticism of the all-academy proposal comes as Conservative councillors raised concerns, calling on the government to reverse its plans for England’s schools.
Earlier this week the NUT staged a series of protest rallies against academy status with the rally in London addressed by Labour’s shadow education secretary, Lucy Powell.
Liberal Democrat leader Tim Farron has sent an open letter to teachers attending the NUT conference, also opposing the academy plans.
“I have never understood why the government think that converting a school to an academy will automatically drive up standards. They’re wrong.
“What drives up standards is a well-funded education system and a teaching workforce who receive proper support and recognition for their work.
“My fear is that this policy is worse than misguided – it is downright harmful.”
But schools minister Nick Gibb said that Mr Corbyn’s intervention showed that Labour would “reverse the remarkable rise in standards across England’s schools since 2010, which has seen 1.4 million more young people in schools rated good or outstanding”.
“While Jeremy Corbyn and Lucy Powell are willing to undo the academisation process which started under Tony Blair and Andrew Adonis, we are determined to create a dynamic school-led system which empowers pupils, parents and school leaders.
“It is clear that only the Conservative party will put the interests of children and parents ahead of that of vested interests. Labour should never be trusted to run our schools again.”
The two biggest teachers’ unions, the NUT and NASUWT, are beginning their Easter bank holiday conferences.
On Saturday, Ms Morgan will be putting the government’s case at the NASUWT conference in Birmingham.
As well as debating the plans for more academies, the teachers’ conferences will be debating issues including excessive workload, baseline testing, the misuse of social media and teacher shortages.