If the UK is serious about providing a good education for the nation's children then it must be willing to pay for it, headteachers have warned.
The general election will be a "tipping point", the National Association of Head Teachers (NAHT) said, with school leaders concerned that standards will fall if a new government is unwilling to give more money to education.
Ministers have previously said that school funding is at record levels, and this will increase further over the next two years as pupil numbers rise,
But NAHT general secretary Russell Hobby accused the current government of "flatly refusing to admit the reality" of the pressures schools are facing.
Speaking as the union met for its annual conference in Telford, Mr Hobby said: "Ministers should no longer be allowed to claim that school funding is protected.
"To make ends meet school leaders will be forced to make staff redundant, cut subjects, increase class size and cut back on extra-curricular activity.
"On the steps of Downing Street the Prime Minister promised a country that works for everyone. That begins with our children.
"If we're serious about that as a nation, we need to be prepared to pay for it. June 8 is a tipping point.
"What worries school leaders is that if we have a government that is unwilling to fund education fully and fairly we'll see a domino effect where progress stalls and standards fall in every school."
The union said its own research had shown that two thirds of school leaders (66%) say they are cutting hours for teaching assistants, while just under a third (31%) are reducing staff hours.
Mr Hobby added: "All schools are operating under unacceptable levels of financial pressure. This is a result of the government's choice to freeze spending and keep it at 2010 levels for each pupil.
"Rising costs and additional expenses like increases in National Insurance and pension contributions and the apprenticeship levy mean that the 2010 cash isn't going as far as it used to.
"You can't expect it to. But the government is flatly refusing to admit the reality. Until they do, all schools are at risk."
Anne Lyons, head of St John Fisher Catholic Primary, Pinner and NAHT president-elect said: "Real-terms government cuts will reduce the breadth of opportunities I can offer my children.
"Arts, music and sports are essential parts of school life. Schools in my local authority face savings of more than £18 million by 2019.
"Without sufficient funding, music, the most wonderful experience I offer our children, will have to go.
"The Treasury's short-sighted attitude to national finances is unfair on the children we serve and is already putting school standards in jeopardy."
Research published last month calculated that secondary schools could lose the equivalent of six teachers as a result of severe funding cuts.
The average secondary in England is facing losses of almost £300,000, while primaries will lose out on tens of thousands of pounds, according to the Education Policy Institute (EPI) study.
A recent report by the National Audit Office has warned that schools will have to save £3 billion by 2019/20.
In her speech to the conference, NAHT president Anne Lyons is expected to say budgets are at breaking point, with her own school needing to find £200,000.
"I don't know about you but that kind of cash isn't exactly lying around in reserves," she will say.
"It isn't ready to be simply wiped off the balance sheet just by buying my toilet rolls from a different company.
"I'm not going to find it by hosting a bake sale, or charging parents a fiver to attend a school play, or getting my minibus sponsored by the local Takeaway.
"And yet, this is what The Treasury reckons will solve the problem."
She will add: "The future, partly thanks to Brexit, has never been more uncertain for young people.
"We should be investing in schools in order to provide high quality staff and resources and thereby giving our children the best start in life that money can buy."