Hundreds of headteachers have skipped class for an "unprecedented" march on Westminster demanding increased funding for schools.
School leaders from across England met in Parliament Square before marching to Downing Street where a letter was delivered demanding more cash.
The protest, taken by campaign group Worth Less? rather than a union, saw 1,000 heads take the day off work to ensure their voices were heard.
The headteachers warned of bigger class sizes, reduced subject choices, cuts to teaching staff and a loss of pastoral services and support for the most vulnerable pupils amid budget pressures.
In July, the Institute for Fiscal Studies (IFS) concluded that total school spending per pupil has fallen by 8 per cent in real terms in England since 2010 - at a time when pupil numbers have been increasing.
But the government has said that schools are receiving more money than ever before.
The letter – to be delivered to Chancellor Philip Hammond – warned that school budgets “remain in crisis” and there were not enough staff for headteachers to do their jobs properly.
“Our concerns are so serious that headteachers have travelled in massive numbers from all over England to deliver our message, in person,” school leaders said. “The situation is unsustainable.”
The letter added: “Our schools remain under enormous duress and we cannot sit idly by as the children and families that we serve receive such an unsatisfactory deal from the government."
Angela Rayner, Labour’s shadow education secretary, said: “This unprecedented action by headteachers is a clear sign of the desperate struggle they now face to provide a decent education while balancing the books.
“It is time that ministers listened to the clear message from communities across the country that they have had enough of cuts to their schools.
“The next Budget is their chance to stop the tax giveaways for big business and the banks, and invest in our children’s future instead.”
A Department for Education (DfE) spokesperson said: “There is more money going into schools than ever before, rising to a record £43.5bn by 2020 – 50 per cent more in real terms per pupil than in 2000.
“The OECD has recently confirmed that the UK is the third highest spender on education in the world, spending more per pupil than countries including Germany, Australia and Japan.
“Every school attracts more funding per pupil through the National Funding Formula, high needs funding has risen to over £6bn this year and the 3.5 per cent pay rise we announced for classroom teachers on the main pay range is backed by £508m government funding.
“We know that we are asking schools to do more, which is why we are helping them to reduce the £10bn spent each year on non-staffing costs, providing government-backed deals for things like printers and energy suppliers that are helping to save millions of pounds.”
