The Minster School, Southwell, is facing a £166,000 per year funding cut, if a new government formula is approved.
The head, Mr Matthew Parris, said the proposal caused deep concern and urged parents to oppose the changes.
He said: “The concept of the new formula was to address the rebalancing of underfunding in parts of the country, such as Nottinghamshire, but, as a result, schools such as the Minster will drastically see a reduction in their funding.
“I do not disagree that some schools will receive additional funding. It is laudable that there will be more support for vulnerable people, but that is taking money away from other schools.
“Despite the fixed income, every year there are incremental salary increases and this has been considerably exacerbated by the government’s increase of National Insurance contributions to be made by employers and increased pension contributions.
“In real terms I and the governors have already contended with approximately £125,000 annual staff cost increase, and further increase of £220,000 in pension and National Insurance contributions, bearing in mind these are annual costs to be met.”
Last year the school made nine redundancies, which meant the average class size in the lower school was 30.
Mr Parris said the school had worked hard in recent years to balance the books, because funding has been frozen since 2014-15.
Mr Parris has raised the issue with Southwell MP Mr Robert Jenrick.