Labour, the Liberal Democrats and education unions have turned their fire on the Tories over school funding as the classroom became the latest election battleground.
Jeremy Corbyn said the Conservatives had been "starving schools of funding" as he promised to hike corporation tax to pay for a cash injection into the education system.
The Liberal Democrats promised a £7 billion spending boost on schools, while the headteachers' union said the June 8 General Election was a "make or break moment" for education.
To fund Labour's plans for a National Education Service, corporation tax would rise from its current 19% level to 26% by 2021/22, with the Opposition claiming it would raise an extra £20 billion a year.
There would be a real terms increase in funding, with £4.8 billion extra going to English schools each year by the end of the parliament.
Mr Corbyn, who will launch the policy in Yorkshire, said: "People of all ages are being held back by a lack of funding for education, and this in turn is holding back the economy by depriving industry of the untapped talent of thousands of people.
"The Conservatives have spent seven years starving schools of funding, meaning headteachers are having to send begging letters to parents to ask for money.
"They have also cut support for students and forced colleges to increase fees. It's created a downward spiral that is bad for the people being held back and bad for the economy."
The Daily Telegraph reported that critics of Mr Corbyn could launch a fresh bid to oust him if he attempted to cling on after a June 8 defeat.
The newspaper claimed as many as 100 MPs could resign the Labour whip in Parliament and sit as a "progressives" group in an effort to force Mr Corbyn out.
An unnamed senior Labour source told the Telegraph there would be a "bloodletting" when Parliament returned in July and "sensible people are going to walk away".
A spokesman for Mr Corbyn dismissed the report as "another silly story from a Tory paper and anonymous source to distract from Labour's policies for the many not the few".
The Lib Dems promised a five-year package to reverse cuts to frontline education budgets.
Party leader Tim Farron, who will continue his battlebus tour of Lib Dem targets in south-west England, warned that a landslide victory for the Conservatives would allow Mrs May to "cut our schools to the bone".
The Lib Dem education plan includes £3.3 billion being spent on protecting per pupil funding in schools, with £660 million used to do the same in colleges.
A further £1.26 billion would go to ensure no school loses out from the national funding formula.
The National Association of Head Teachers' general secretary Russell Hobby said: "This General Election is a make or break moment for education. School budgets are at breaking point right now.
"More money for schools is absolutely vital. Whoever forms the next government needs to fund education fully and fairly, reversing the £3 billion of real terms cuts schools are facing."
Tory Treasury minister David Gauke said: "Jeremy Corbyn can't deliver any of this - they're just made up promises on the back of nonsensical spending plans.
"He's spent this damaging tax rise on businesses on 12 different things and he's already dropped numerous things he's said he'd do before.
"The Lib Dems are no better and won't even tell people about the tax rises they would bring in."
Mrs May will focus on her Prime Ministerial duties in Number 10, where she will meet Nato general secretary Jens Stoltenberg, before heading out on the campaign trail in the East Midlands.
Conservative campaign chiefs will be hoping that voters have warmed to the PM following her appearance alongside husband Philip in a highly personal interview on BBC's One Show.
Mr May said it was "humbling" that people are "entrusting their futures" to his wife, while the Prime Minister revealed she got him to do the "boy jobs" around the house, such as taking out the bins.
City financier Mr May revealed that his wife had first set her eyes on Number 10 while on the frontbench in opposition, indicating she could have been plotting her leadership bid as far back as 1999.