Jeremy Corbyn has pledged to plug a £3bn black hole in school finances in England.
The Labour leader's promise that education would be a spending priority for his party came at a head teachers' conference in Telford.
He hinted that he would use corporation tax to pay for the extra spending commitments but said specific details would be in Labour's manifesto.
The Conservatives insist school funding is protected and at a record high.
The National Audit Office says schools face £3bn in spending pressures by 2020 and head teachers around the country have been looking at making staff redundant and cutting provision.
Mr Corbyn's speech comes a day after National Association of Head Teachers (NAHT) heads warned that schools may be forced to opt for a four-and-a-half day week to plug their budget shortfalls.
Mr Corbyn was asked by one delegate, John Gadd, a head teacher from West Sussex, if he was "brave enough to fully reverse the £3bn in cuts".
He replied: "I believe we are brave enough to do it because I see education as a complete priority and that is what I want to achieve.
"We've got to look again at the spending we put into schools and if every school is now faced with a funding crisis, or the vast majority of them, that is not a good way forward, and I am determined to lead a government that will give the priority it deserves to education, not the blame culture of head teachers and teachers in under-funded schools."
'Core budget protected'
He suggested he would be using changes in corporation tax to tackle schools funding pressures.
He told head teachers: "While funding to our children's education is cut, multinational corporations have received multi-billion pound tax giveaways.
"How can it be right that money is being siphoned straight out of our children's schools and directly into the pockets of the super-rich?"
The Department for Education has previously said that it has protected the core schools budget in real terms since 2010, with school funding at its highest level on record at almost £41bn in 2017-18 - and that is set to rise, as pupil numbers increase over the next two years, to £42bn by 2019-20.
A spokesperson said: "We recognise that schools are facing cost pressures, which is why we will continue to provide support to help them use their funding in cost-effective ways and make efficiencies."