Head teachers at seven schools in Sheffield have written an open letter to parents urging them to fight budget cuts of 20% for sixth form education.
In the letter the heads tell parents: "We have never been subjected to cuts of this magnitude."
The letter claims class sizes may increase and some subjects may be cut over the next three years.
The Department for Education said it was moving to a fairer and more transparent funding system.
The letter, signed by the heads of King Ecgbert, Tapton, Silverdale, All Saints' Catholic High, High Storrs, King Edward VII and Notre Dame Catholic High, tells parents some classes may be taught in less time and make more use of ICT for delivery.
The heads also said that some subjects with low class numbers may be scrapped and that "guidance and enrichment" sessions may be reduced.
Parents are urged to contact their MP and write to Michael Gove, the Education Secretary.
David Bowes, the head teacher of Tapton School said there had been concerns about funding for post-16 education under the previous government.
"There was a notion that this would be harmonised and we were delighted about that because we had always thought our colleagues in Sheffield College for instance were under-funded," he said.
"But what has happened with a change of government, a new administration, a notion of fairness has been brought in.
"That is not that colleges would rise to the level of funding that sixth forms have, but that the reverse would occur and schools with sixth forms would be harmonised and our funding would come down to meet theirs."
In a statement the Department for Education said their aim was to make the funding process fair.
The department said: "We are moving to a fairer and more transparent funding system for further education.
"For years, school sixth forms have been funded at on average £280 more per pupil than further education colleges - for a medium sized college this would be the difference of around £500,000.
"In the current economic climate it does not make sense for this inflated funding to continue so we are moving to a level playing field.
"We have put in place transitional funding arrangements to help school sixth forms adapt."