Class sizes are set to increase and the range of subjects offered to pupils will be reduced due to a lack of funds, a head teacher has said.
Chas Drew, head at Commonweal School, Swindon, also confirmed that the number of support staff for children with special educational needs will be reduced and help for low-income families for uniform is being cut.
He added: "I'm not saying these things publicly because I'm trying to cause an argument, I just need to be honest with our community and I think our political leaders need to be honest with the country."
The government said it has invested a further £3.2bn in schools and it will support leaders to use funding efficiently.
The secondary school, which has 1,400 students, will stop teaching health and social care and sociology. Pupils half way through their GCSEs will be able to continue with the subjects.
Mr Drew continued: "In core subjects, we having to make groups bigger, have more children in each class. That's because we're having to reduce our number of teachers from 94 down to 90.
"That doesn't sound like a huge change but when you've got no slack left in the system, that has a really significant impact."
The letter said the school also plans to reduce numbers of support staff, cut the amount of alternative provision for special educational needs children and slash help for those on low income for trips and uniform.
He said it is a national problem with costs rising more than income ever year. He called for a "grown-up conversation" between schools and government.
