Olchfa comprehensive in Swansea is seeking Government approval to reserve up to 28 Year 7 places each year for pupils who excel either academically or at sport. If its bid is accepted it will become the only school in Wales to use selection to determine part of its intake.
Governors at the school insist the plan will only come into operation if demand for places outstrips supply but fears were expressed this week that the thin end of a long wedge is being sharpened.
In academic terms the 1,840-pupil school is rated among Britain's best comprehensives, with 66 per cent of 16-year-olds achieving five or more GCSEs last year.
Competition to get into Olchfa is so intense that parents move into the catchment area. Others have registered their unborn children for a place. With an average intake of 280 pupils a year, the school is heavily oversubscribed.
The governors, chaired by Swansea solicitor James Tonner, deny that the proposed changes in admission policy represent a move towards selection and say that pupils in the catchment area would continue to be admitted on a comprehensive basis. They said in a statement: "This proposal is still at a very early consultative stage."
The shift in policy has caused controversy throughout Wales, which has a long tradition of support for the comprehensive system. It coincides with a testing time for the school with the headteacher, John Booth, due to retire this term.
No permanent replacement has been appointed although there have been applications from 16 contenders, including three of the school's four deputy heads. One of the deputies, Stewart Wainwright, is to take over as acting head from September with Mr Booth staying on as consultant head until a decision has been made.
Peggy George, the National Union of Teachers county secretary in West Glamorgan who retired as Olchfa's deputy head in 1988 is perplexed: "It seems totally inappropriate to reconsider admission procedures at this time."
Alun Jones, Wales regional officer of the National Association of Schoolmasters Union of Women Teachers, predicted that there would be objections from parents matching the rejection of the school's bid for grant- maintained status three years ago, which was given the thumbs-down by more than two to one in a ballot of parents.
"It will be thrown out because it allows for some form of selection and that goes right against the grain in Wales."
The controversial move from Olchfa comes at a time when governing bodies are considering the Popular Schools Initiative, launched at the end of last year by John Redwood, the Welsh Secretary.
Under the scheme, Pounds 23 million is being made available over the next four years to fund expansion in schools which attract more pupils than they can accommodate.