More than 5,000 of the nation’s most hard-pressed schools could be denied any extra cash for the next ten years.
That means heads will either have to fire teachers, drop courses or cut school hours, and some grammars are warning parents might have to pay to make up shortfalls.
Education Secretary Justine Greening is introducing a new funding formula billed as providing a safety net to stop the poorest schools going under.
But the National Union of Teachers says these safeguards are not all they seem.
It says 5,333 schools, a fifth of the nation’s total, will be given flat cash settlements to struggle along with no extra in sight until 2027.
Yet they will still face additional costs such as higher employer pension and national insurance contributions.
The NUT says the new formula will make 98 per cent of schools worse off in real terms by 2020.
And that means schools will have to make eight per cent savings, worth a total of £3billion.
The NUT claims that under present plans the poorest schools will see no extra funding for another seven years after that.
Head teachers hoped Chancellor Philip Hammond would reverse the cuts in Wednesday’s budget.
And although he pledged £216million for refurbishments, £320million went to building 110 new free schools, some of which will be grammars.
NUT general secretary Kevin Courtney said: “This budget is a complete dereliction of duty to our children. The Chancellor knows full well that schools are on their knees.
“Extra funding will pour into new free schools and grammar schools – for which there’s absolutely no need.”
Lib Dem Education spokesman John Pugh added: “This is totally the wrong priority for schools funding.
“The free schools programme has overspent while existing schools struggle to pay for books, cut teachers, and their buildings decay around them.”
Russell Hobby of the National Association of Head Teachers said: “For many schools, this Budget was their last chance.”
School governors are even threatening to strike rather than sign off underfunded budgets.
Unions are now eyeing up the apprenticeship levy which will raise £2.8billion from employers by 2020.
Only £640million of that is earmarked to be spent on apprentices so there would be money in the pot to help the most beleaguered schools.
A Department for Education spokesman said: “The union’s figures are fundamentally misleading.
“As pupil numbers rise, the amount of money schools receive will also increase. School funding is at its highest level on record.
“Under the proposed national schools funding formula, more than half of England’s schools will receive a cash boost.”