"It's a confidence trick," he says. "GM [grant-maintained] schools will resist any attempt to just turn them into local authority schools under another name."
Langley Park opted out in 1991 because its governors felt it had been neglected by the local council. A pounds 3.5m building project had been cancelled and many of the 650 boys were being taught in temporary classrooms.
Since then bids for money from the Funding Agency for Schools have also been unsuccessful, though Langley Park did have pounds 100,000 to improve its technology facilities - a sum matched by other local schools, Mr Sheffield says.
But it has also been able to manage its own funds better, employing a full-time groundsman at less cost than a previous contract which had failed to satisfy, and it has become ever more popular. Last year there were 650 applications for 180 places.
Mr Sheffield fears that under Labour's scheme Bromley could force his school to take extra pupils - something it has always resisted.
He does not think the borough would object to its selection of 18 boys each year by musical aptitude, though he believes such specialisms might be frowned on in other areas.
"What it has done is to enable me to do things such as increase the staffing in the school. It has enabled me to increase resources and that has enabled us to considerably increase the academic performance of the boys."
He predicts that the GM sector will not go down without a fight: "If we are dissatisfied with the proposals and if Labour is elected, it will be up to us to mobilise support amongst our parents to act as a pressure group to try to influence the passage of legislation through Parliament."Reuse content