Nearly three-quarters of school leaders believe their budgets will be “unsustainable” by 2019, according to a survey of headteachers.
The poll of 1,102 school leaders reveals 18 per cent of schools are in deficit, up from 8 per cent last year.
More than two-thirds (71 per cent) of school leaders say they are only able to balance their budgets by making cuts or dipping into reserves.
Nearly three-quarters (72 per cent) also say their budgets would be unsustainable by 2019.
The survey reveals that 85 per cent of respondents are having to reduce investment in equipment.
Increases in payroll costs, as a result of government policies, were cited as schools’ biggest financial pressure.
These costs went up in 2015 and have resulted in an increase to school budgets of over 5.5 per cent every year, but there has been no resulting increase in funding from the government, according to headteachers’ union the NAHT, which carried out the survey.
Nearly half of respondents say the decline of local authority services has resulted in cost pressures.
In particular, they referred to the abolition of the education services grant (ESG), which funds school-improvement services and is being phased out this year.
Nearly two-thirds (65 per cent) of academy school leaders say they are concerned about the impact of cuts to the ESG.
The responses relate to schools’ 2016-17 budgets.
Bernadette Hunter, headteacher at William Shrewsbury Primary in Burton-upon-Trent, said: “Costs are rising at a time of stagnant budgets, and the new funding formula that we hoped would help will see our school lose £38,000 – a teacher’s salary. This is devastating.
“The funding formula will fail if there is not enough money put into it and children's learning will suffer.”
Liam Collins, headteacher of Uplands Community College in East Sussex, said: “With inflation and unfunded cost increases, we only have £4,431 to spend on each pupil. To our overall budget, this is a cut of £208,711, which in three years rises to our school being underfunded by £350,000.
“In simple terms, this is a cut of 10 teachers, fewer clubs, no pastoral support, a narrowed curriculum, no counselling for students struggling with mental health issues, crumbling buildings, no IT upgrades, no new textbooks and no school planners."
The growing number of children with mental health issues is a big concern for members, said the NAHT.
The cost of dealing with pupils’ additional needs was the third most quoted source of financial pressure, reported by 83 per cent of respondents.
NAHT general secretatary Russell Hobby said: “School budgets are being pushed even closer to breaking point than before.
“Schools are acutely feeling the impact of an estimated £3 billion shortfall in the government’s education budget by 2020 – the first real-terms cuts to education spending since the 1990s.”
He said 98 per cent of schools are facing cuts and called for the government to take "urgent action".
A Department for Education spokesperson said: “We have protected the core schools budget in real terms, so that school funding will be over £40 billion in 2016-17 – its highest level on record.
“We are also consulting on plans to end the disparity in the school funding system. These proposals will not only see more than half of England’s schools receive a cash boost in 2018-19, but will also give headteachers certainty over their future budgets, helping them make long-term plans and secure further efficiencies."