Funding cuts have forced secondary schools to shed 15,000 staff over two years – as pupil numbers have risen by more than twice that amount, according to unions.
An analysis of government data by the School Cuts coalition of education unions shows that in the two years from 2015-16, when per-pupil funding levels started to fall, secondary schools have lost 5.5 staff members on average.
This equates to 2.4 fewer classroom teachers, 1.6 fewer teaching assistants and 1.5 fewer support staff.
The coalition says this has happened at a time when pupil-to-classroom teacher ratios are rising, which means bigger classes and less individual attention for children.
Geoff Barton, general secretary of the Association of School and College Leaders, said: “This is most damaging to children who need extra support and whose parents cannot afford to supplement their education with activities outside school.
"If the government is to live up to its promise to improve social mobility, it must give schools the funding that they need.”
The average figures mask significant regional variations, the analysis shows. And some of the largest staffing cuts are in the areas with the lowest average funding per pupil, including Reading, the Isle of Wight, Central Bedfordshire, the East Riding of Yorkshire, York, Derby and Milton Keynes.
The coalition says the situation is likely to get even worse, as it predicts that nine out of 10 primary and secondary schools in England and Wales will be hit by a real-terms cut in funding per pupil between 2015-19.
Teacher numbers cut
Mary Bousted, joint general secretary of the National Education Union, said: "Schools are cutting back on teacher numbers and the pupil-to-teacher ratio is worsening. Children only have one chance to go to school. We should be investing in this generation of young people who will see such profound changes during their lifetimes.
"Ensuring schools have sufficient funding to educate our children properly must become the top priority for education secretary Damian Hinds.”
Additional funding announced last summer – raided from schools' capital budget – will result in per-pupil spending being frozen in real terms between 2017 and 2019, according to the Institute for Fiscal Studies. However, schools will still see a real-terms cut of 4.6 per cent between 2015 and 2019, due to cuts made before the cash boost.
These cuts came after years of years of real-terms rises; per-pupil funding increased by 82 per cent over 20 years in secondary schools, while it more than doubled in primaries.
But Paul Whiteman, general secretary of the NAHT heads' union, said school budgets were now "at breaking point".
"School leaders have made every other possible efficiency and now it is impossible for many schools to avoid making redundancies, to continue to keep class sizes at an acceptable level, and to offer a full and rounded curriculum to all pupils," he said.
The Department for Education has been contacted for comment.