The National Association of Head Teachers (NAHT) surveyed 1,069 schools and found 64 per cent of 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 seven per cent of those surveyed were already running at a loss, the union’s Breaking Point report showed.
NAHT spokesman Russell Hobby said school budgets were already overstretched and the costs for national insurance and teachers’ pensions would increase by more than 5% from this school year.
He said: “The money coming into schools is not keeping up with the expenditure they face.”
Mr Hobby added: “This is coming right at the time when we’re expecting more and more of schools.
“So we’ve got hundreds of thousands more pupils coming their way, we’ve had years of very little maintenance on the buildings, and we’ve got much higher standards as well.”
Close to half of the school leaders surveyed said their budget would be untenable within two years and 67% believed they would be unable to balance the books in four years.
The review also found over 80% of school leaders feared education standards would be negatively affected by their cuts.
A Department for Education (DfE) spokeswoman insisted the Government was protecting the schools budget, which would rise as pupil numbers increase.
She said: “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.”