Schools are facing financial "breaking point" as figures show the number in deficit has doubled since 2015, teaching chiefs have warned.
The National Association of Head Teachers (NAHT) said its survey of 1,102 school leaders found widespread concern about budgetary pressures.
Nearly three-quarters (71%) of respondents said their school was forced to tap into reserves and resort to spending cuts to make ends meet.
Similarly, 72% said their budgets will be "unsustainable" by 2019, NAHT said.
A sharp rise in the number of schools entering deficit was also found in the 2016/17 budget survey - up 10 percentage points to 18%.
NAHT general secretary Russell Hobby said: "School budgets are being pushed even closer to breaking point than before.
"Schools are acutely feeling the impact of an estimated £3bn shortfall in the Government's education budget by 2020 - the first real-terms cuts to education spending since the 1990s.
"The Government must take urgent action and commit to funding schools sufficiently in the next budget. It is time to stop viewing education spending as a cost and to start seeing it as an investment in England's future, and in our children's."
Chief among the concerns of school leaders were spiralling payroll costs, fuelled by rises in pensions, national insurance contributions and an apprenticeship levy, which has led budgets to shoot up by 5.5% per year, it is claimed.
A knock-on effect from cuts to health and social care funding was also reported, with nearly 79% of respondents saying they had to provide direct support to children with mental health issues.
Bernadette Hunter, head teacher at William Shrewsbury Primary in Burton-upon-Trent, said: "The removal of the Educational Services Grant, wage inflation, increases in pension and National Insurance contributions, the apprenticeship levy and cost of living increases, are all leading to a real terms cut for schools.
"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."
Labour's shadow education secretary Angela Rayner said: "The Tories can continue to hide their heads in the sand, but the school funding crisis is real and it is already hitting schools.
"Headteachers should not be faced with a decision of whether to cut school staff or invest in new equipment.
"The Tories have no plan to deal with falling budgets, chronic teacher shortages and not enough good school places - this is no way to run a schools system."
A Department for Education spokesman 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.
"We recognise that schools are facing cost pressures, which is why we will continue to provide advice and support to help them use their funding in cost-effective ways, including improving the way they buy goods and services, so they get the best possible value for their pupils."