Education spending must be increased to prevent staff cuts because of a "funding crisis", two teaching groups have said.
In an open letter to the Chancellor, the National Association of Head Teachers (NAHT) and the National Governors' Association (NGA) warned schools are running out of things to cut.
NAHT general secretary Russell Hobby and Emma Knights, NGA's chief executive, called on Philip Hammond to use the March Budget to provide "desperately" needed funding.
They wrote: "We are writing to underline the urgency of the school funding crisis and to ask you to use the upcoming Budget to deliver the investment that schools so desperately need.
"Governing boards and school leaders are being forced to make impossible choices as a result of insufficient funding.
"They are doing their best to 'make do', but there are only so many financial efficiencies a school can find before reaching breaking point. Schools are running out of things they can cut.
"For many schools, the only real way they can make the savings the Government is asking for is by making staff redundant."
They urged the Chancellor to stick to the manifesto pledge to protect school funding.
"Every young person's experience of school matters because it is an investment in the future of our society and economy," they added.
"Greater investment in schools now will mean future costs associated with poor health, crime and unemployment are likely to be lower."
Headteacher Mary Sandell said she is quitting because of the funding shortfall.
She told BBC Radio 4's Today programme: "I'm going at the end of the summer holidays, August 31.
"I'm leaving something I love because quite simply there is not enough money to do the job properly.
"It means fewer subjects at A-level, fewer subjects at GCSE, more children in classrooms and overall fewer teachers."
Ms Sandell, of Forest School in Winnersh, Berkshire, said she understood her school was due to receive a 2.9% funding increase.
But she said: "We know that we cannot replace equipment, we cannot improve the buildings, we can't even afford textbooks for new specifications. That's now.
"Even if we were to get 2.9%, and I think that's unlikely, it will not compensate for years of underfunding, because Wokingham has been the lowest-funded borough in the country for years and years.
"I've been in education since 1980 and it's the worst it's ever been from my perspective."