Labour and the Lib Dems are both making multibillion pound funding pledges to tackle school cash shortages in England.
Labour says it would raise corporation tax to spend £4.8bn on keeping up with rising costs, and £335m so no schools lost out from a funding reorganisation.
The Lib Dems are promising £7bn to protect per-pupil funding levels.
The Conservatives said school spending had reached record levels and attacked the two parties’ spending plans.
Head teachers have been warning about job losses caused by cash shortages, with the National Audit Office estimating schools will have to cut £3bn from budgets by 2019-20 to meet rising cost pressures.
A new funding formula will also see some schools lose money, although others will gain.
On Monday, the Institute for Fiscal Studies said protecting schools from real-terms cuts in England would mean spending the equivalent of an extra penny on the basic rate of income tax.
Its director Paul Johnson said Labour would be able to fund the plan by raising corporation tax by 19% to 26% by 2021. But he told the BBC this would amount to one of the single biggest tax rises for about 30 years.
Labour leader Jeremy Corbyn and shadow education secretary Angela Rayner are promising a real-terms funding increase in schools and “free lifelong education” in colleges, including restoring the Educational Maintenance Allowance scrapped in 2010.
Ms Rayner told BBC Breakfast the Conservatives had “broken their contract” with the public over school funding, to the point that some schools were having to let teachers and teaching assistants go.
“I want to reassure parents up and down the country that my priority is that our education system will be fully funded,” she said. “Not only will schools have the money they need to ensure schools reach their full potential but adult education will be free at the point of use so people can go back into education.”
She said this would meant anyone who wanted to re-train, for example, as a nurse would be able to do so.
Asked why Labour was not talking about tuition fees and whether the party was ruling out scrapping them, she later told BBC Radio 4’s Today “watch this space” – adding that ahead of its manifesto launch next week Labour was focused on ensuring its policies were cost and could be delivered.
The Lib Dems claimed their plans went further than Labour’s and said details of how they would be funded would be provided “in due course” in the party’s manifesto.
The party said it would also protect further education per-pupil funding in real terms.
Lib Dem education spokeswoman Sarah Olney said: “While funding per pupil is set to see the biggest cuts in a generation, billions of pounds are being spent on divisive plans to expand grammars and free schools.
“This extra £7bn of funding would ensure no school and no child loses out.”
The Conservatives said Labour had already committed the corporation tax rise to other projects.
Chief Secretary to the Treasury David Gauke said Labour’s announcement was “just made-up promises on the back of nonsensical spending plans”.
He added: “The Lib Dems are no better and won’t even tell people about the tax rises they would bring in.”
BBC education editor Branwen Jeffreys says schools in England have been experiencing a financial squeeze for the first time in 20 years. Plans for the different funding formula, which have caused concerns among some Tories, have galvanised lobbying by head teachers and parents, she added.
As education is a devolved matter, the pledges relate to England only, and the amount Scotland, Wales and Northern Ireland receives would be determined by the funding formula.
Read more at BBC.co.uk