Whitchurch High School would be able to manage its pupil intake
The biggest school in Wales, which is facing possible cuts in pupil numbers, is seeking to manage its own affairs.
Whitchurch High School in Cardiff is to apply for foundation status, which would free it of local council control.
If approved, it could scupper council plans to reduce numbers at the school, which has more than 2,260 pupils.
But the application would have to go out to consultation and may take at least a year, with the final decision taken by Education Minister Jane Hutt.
And one union said it was deeply opposed to the plan and could not rule out a ballot on industrial action.
As a foundation school, Whitchurch would be able to manage its own admissions policy at a time when the council is looking at options to reduce pupil numbers.
"We are proposing to go from 12 to nine forms," said a spokeswoman for Cardiff council.
"If that takes place and Whitchurch then gain foundation status they would be able to increase back up to 11 forms."
The council is trying to tackle the problem of surplus school places in the city which it says costs £3m a year.
Across Cardiff, the council says there are 8,620 surplus places.
Cardiff North Conservative AM Jonathan Morgan said he supported the school's application.
"I think it's a terrific idea," he said. "It would give the school flexibility to manage its own pupil intake."
The public services union Unison said it was deeply opposed to the move towards foundation status.
"We have called a meeting of our members in Whitchurch and will be consulting with them, but as a union we are completely opposed to this move and cannot rule out a ballot for industrial action if the governors do not reconsider," said Mark Turner, branch secretary of Cardiff county UNISON.
There are currently eight secondary and four primary foundation schools in Wales.