Class sizes have increased and vulnerable children are not getting the support they need in Peterborough because of funding cuts, a headteacher has warned.
The area's Tory city council wrote to schools to ask them to explain how increasingly tight budgets were affecting them.
The results - which will be discussed by councillors next week - paint a shocking picture which headteachers and officials have called "demoralising".
Bryan Erwin, principal at Ken Stimpson Community School, said his pupils were losing out.
He has already had to cut back on teaching assistants.
Across the city, subjects like drama have already been cut, as have therapy sessions for some of the most vulnerable pupils.
Teachers have found they have less money to spend on essential equipment and heads are teaching classes to make up for staff shortages.
The survey of headteachers was commissioned by Jonathan Lewis, Peterborough City Council's Director of Education.
He said although schools were receiving more money from the government, there were far more pupils in need of places and that meant headteachers were grappling with real-terms cuts of around 10 per cent of their budgets.
And in Peterborough, those numbers are often worse - with the area seeing the fifth fastest population growth in the country. Mr Lewis said many schools in the city had seen their budgets drop by 20 per cent.
In a statement, the Department for Education insisted funding for schools had increased by 2.3 per cent per pupil this year - amounting to an extra £12.5m when rising pupil numbers were taken into account.
But it acknowledged schools were facing challenges with their budgets.
For Lynne Ayres, cabinet member for education at Peterborough City Council, that does not go far enough.
She said she would be demanding a meeting with ministers to discuss the findings of the survey.
The Conservative councillor said she had been impressed with the way the area's headteachers and teachers had found innovative ways to deal with an increasingly small pot of money.
But she said austerity had gone too far and she could see that funding cuts were now disadvantaging pupils and affecting teachers' welfare.