Wallington county grammar in Surrey, a grant-maintained boys' school, faces cutting staff, exam courses, free musical tuition and extra-curricular sport and could also be forced to ask pupils to pay for their own textbooks.
It follows a progressive annual drop in the school's budget - pound;300,000 less this year than five years ago. Wallington says the cash would prevent massive cuts which could "destroy the nature of the school".
Head Martin Haworth said: "Financially, the scenario is very depressing. In common with other grant-maintained schools, our budgets are being reduced.
"This year, in particular, there has been a noticeable fall and we expect further loss in 1999.
"We do not usually go to parents cap in hand, we have simply tightened our belts over successive years. However, at this stage we thought it would not be inappropriate to ask for an increased contribution to the school fund.
"We enjoy the support of parents and do not expect there to be any great concern about our request for a contribution."
In a letter to parents, Dr Howarth blames the crisis partly on "inequitable funding" of local education authorities by central government and partly on the local funding formula for schools "which is structured to the disadvantage of local grammar schools".
Wallington opted out five years ago and was praised by OFSTED for giving value for money.