The decision follows allegations that governors did not give parents adequate opportunity to hear the case against grant-maintained status.
Earlier this month, parents of pupils at Trinity Roman Catholic comprehensive voted five to two in favour of opting out. However, after complaints from the Association of Teachers and Lecturers as well as several parents, the Department for Education and Employment has announced it is to carry out a "thorough" investigation into the conduct of the ballot.
Ralph Surman, Nottinghamshire branch secretary of the ATL, said: "This is the biggest decision Trinity comprehensive has ever taken, but parents were given no chance to weigh up the pros and cons before the ballot."
A spokesman for Nottinghamshire County Council, which is making a detailed submission to the DFEE, said: "The governors gave the authority no opportunity to outline its opposition to grant-maintained schools.
"They made no attempt to organise parents' meetings and, contrary to government guidelines, the letters sent out to parents only outlined the arguments in favour of opting out."
But Bernard Bonner, headteacher of the 835-pupil voluntary-aided school, dismissed the claims. "There was a very strong vote in favour of opting out and there has been nothing untoward in the school's conduct," he said.
"We went out of our way to be even-handed and I can see absolutely no foundation in the complaints, nor any reason why the ballot result should not stand."
Gillian Shephard, the Education and Employment Secretary, can declare a result void if she finds "evidence of irregularities in the conduct of the ballot, or that the outcome was influenced to a significant extent by materially misleading information," according to the 1993 Education Act.