School funding is close to breaking point, the leader of a teachers' union will warn today.
Russell Hobby, the general secretary of the National Association of Head Teachers (NAHT), will say that schools have many challenges ahead of them, but not enough money to face them.
Speaking at the NAHT education conference in central London, he will refer to a recent survey carried out by the union, which found that two thirds of school leaders had introduced belt-tightening measures to stave off deficits.
These included cutting essential maintenance, reducing spending on both teachers and teaching assistance, reducing investment in equipment, and carrying over a surplus.
But 7% of those surveyed were already running at a loss.
"Schools seem to face more of everything except money," he will tell the conference of school leaders, school business managers, and children's centre leaders.
"More pupils, more tests, higher demands, higher costs. But spending is flat, leading to a real terms cut in school budgets over the next few years.
"Two thirds of schools are only staying afloat this year by making cuts or dipping into reserves. The same proportion say their budgets will be untenable before 2020. Four out of five believe standards are already starting to fall.
"Flat cash is not enough if we want to keep improving standards. We need education funding to match the real cost pressures in schools.
"Education is not a cost, it is an investment. The hollow promise of protection is really a cut, and this is nothing for ministers to be proud of."
A Department for Education spokeswoman said: "We are protecting the schools budget, which will rise as pupil numbers increase.
"This government is committed to making sure schools are funded fairly so all pupils have access to a good education - a key part of our core mission to raise standards across the country and make sure every child reaches their full potential.
"We have made significant progress towards fairer funding for schools, through an additional £390 million allocated to 69 of the least fairly funded areas in the country - the biggest step toward fairer schools funding in 10 years."