Nick Clegg will not "tolerate" state-run schools which are run for profit, he has said.
The deputy prime minister's strong stance came in a speech about the education system which addressed the Liberal Democrats' attitude towards the Tory-driven free schools programme.
Education secretary Michael Gove is pushing to turn as many schools as possible into academies, giving them independence from control by local authorities, freedom around the delivery of the curriculum and freedom to change the lengths of terms and school days.
The Department for Education said 1,000 new academies have opened in the last year, with a further 49 due to open during this academic year.
Forty per cent of all secondary schools are now open or in the process of opening as academies.
It had not yet been made clear before today whether the coalition would let private firms investing in them make profits from the enterprise, however.
Mr Clegg laid out his position in a speech in London. "To anyone who worried that, by expanding the mix of providers in our education system, we are inching towards inserting the profit motive into our school system, again, let me reassure you," he said.
"Yes to greater diversity; yes to more choice for parents. But no to running schools for profit, not in our state-funded education sector.
"They must not be the preserve of the privileged few - creaming off the best pupils while leaving the rest to fend for themselves, causing problems for and draining resources from other nearby schools.
"So let me give you my reassurance - I would never tolerate that."
Yesterday Mr Gove sought to brush over the claim that Mr Clegg had prevented him from allowing free schools to make money in the state system.
"Nick and I are completely in agreement on this. It's not an issue," Mr Gove told BBC1's The Andrew Marr Show.
"The Conservative election manifesto said that we don't need to have profit at the moment, the Lib Dem manifesto said that we don't need profit at the moment and we don't."
Mr Clegg also used his speech to take pressure off teachers in a bid to give parents more responsibility.
"We already expect our teachers to be social workers, child psychologists, nutritionists, child protection officers," he will add.
"We expect them to police the classroom, take care of our children's health, counsel our sons and daughters, guide them, worry about them – and on top of that, educate them too.
"When you consider that list, it is phenomenal that so many rise to the challenge. But it is too much to ask. Teachers are not surrogate mothers and fathers; they cannot do it all."