Jeremy Corbyn has told headteachers he would reverse the £3 billion of savings schools will have to make by 2019-20.
Asked at the NAHT conference in Telford whether he was "brave enough" to fully reverse the £3 billion of savings the National Audit Office says schools will have to make, 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 because it unlocks the potential of children."
Asked afterwards to give a "copper bottomed guarantee" that Labour would fund the £3 billion of savings, he said: "We are very clear on that and how it will be done will be in the manifesto."
The Department for Education routinely says that school funding is at record levels.
He said Labour would "halt" the government's proposed national funding formula, which he said often hits the most needy schools in the poorest areas hardest, and "refund it".
Corbyn said he wanted to give councils to the power to take over existing academies "where they want to but they would not be forced to do so".
He said he also wanted councils to have the authority to build a new school if it is needed, and restore the role of local education authorities as "coordinators of local education and local education needs".
Corbyn reiterated Labour Party policy to create a National Education Service, from early years to adult education.
He said: "It's not a gimmick at all. Dealing with 30 children is difficult; dealing with 31 is more difficult; dealing with 40 is very, very difficult indeed.
"Obviously we would all like the smallest class sizes we can get within reason. I think 30 is an absolute limit on it, and that's what I would want to stick to."
Heads applauded when he said he would lift the cap on public sector pay, and fund free school meals for all primary school children.
Asked to describe his education thinking in three words, he said: "Every child special".
Education secretary Justine Greening had been due to address the conference on Saturday, but pulled out after the election was called.
Last year, then-education secretary Nicky Morgan was jeered and heckled by headteachers at the conference.