Schools are considering cutting teacher numbers and making class sizes bigger in a bid to save money.
In a new poll by an education union, most said they were likely to take these measures or were at least thinking about them.
Headteachers have been raising the alarm over stretched school budgets for months, as soaring inflation and staff pay increases push up costs.
Nearly all now face having to make cuts, the new poll by the Association for School and College Leaders found, with 98 per cent of respondents saying they would have to find savings either this year or the next.
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Over half – 58 per cent – said they were considering or likely to reduce teaching staff and increase class sizes to deal with cost pressures.
Meanwhile, 55 per cent said they were thinking about reducing the number of teaching assistants in their school, while around 40 per cent said they were considering reducing curriculum options.
One headteacher told The Independent last month that his secondary school had already cut back on subjects in a bid to save on costs.
Geoff Barton, from the ASCL union, said: “School leaders in this survey use words such as ‘catastrophic’ and ‘devastating’ to describe the financial situation they are facing and the impact on their pupils.
“It is clear that the future is bleak unless the government acts urgently.”
In its survey earlier this month, ASCL also asked whether schools and colleges were considering a shorter week to save on costs.
Out of 630 responses, none said they were considering a three-day week. Seventeen, however, said they were thinking about going down to a four-day week.
Mr Barton said: “No government can claim to be serving the public interest by presiding over an education funding crisis which cuts provision and imperils standards.”
Nick Brook, from the NAHT school leaders’ union, said: “The findings from this survey are alarming and very much mirror what we are hearing from our own members.”
He added: “The school funding crisis is impacting children and their life chances directly, right now. The government has to understand the damage they are doing.”
On Tuesday, a think tank warned secondary schools were facing real-term budget cuts equivalent to the loss of up to four teachers.
Its analysis was based on research which said school spending power would be 3 per cent below 2010 levels by 2024-2025 under the current funding settlement.
Headteachers have told The Independent this term their schools were on a “knife edge” and cost pressures were having a “genuine negative impact” on children’s education.
A Department for Education spokesperson said: “We understand that schools are facing cost pressures which is why we are providing schools with £53.8bn this year in core funding, including a cash increase of £4bn for this financial year.
“All schools will benefit from the Energy Bill Relief Scheme, reducing how much they need to spend on their energy and giving them greater certainty over their budgets over the winter months. We are also providing schools with tools and information to help get the best value for money from their resources.”
