Parents at the Beacon school in Banstead, Surrey, have agreed to help bail the school out of its funding crisis.
But despite 400 families pledging pound;10 a month for the next year, the state secondary could still have to cut one lesson a week and slash teaching posts.
Headteacher John Darker wrote to parents earlier this month asking for help to reduce a pound;300,000 deficit.
Even so, the school will not be replacing nine staff who are leaving, which could result in larger class sizes and a restricted range of courses at GCSE, AS and A-level.
Mr Darker is also having to cut the number of design and technology, music and drama lessons for 11 to 14-year-olds to make ends meet.
He said: "In my 17 years as a headteacher I have not experienced such a desperate situation which has been so directly caused by government failure."