The Government has pledged to introduce legislation "as soon as possible" to abolish GM status but in the meantime the law allowing schools to opt out stands.
Schools standards minister Stephen Byers last week gave the go-ahead for Westoning Lower, a 126-pupil school for four to nine-year-olds, to opt out from July 1.
Pauline Latham, chairman of the Grant-Maintained Schools Advisory Committee, believes that other heads and governors will want to follow suit.
"Schools are still going to get transitional grant and an additional training grant for at least a year. I think schools will seize this opportunity and take it while they can."
Mr Byers approved the opt-out application because of the high level of parental support (it was backed by more than eight out of 10 parents who voted), the lack of local objections and the strength of the governing body.
Somewhat ironically the school will now opt out of an authority that came under Conservative control in the May 1 local government elections.
Mr Byers's decision was coupled with a commitment that the Government would introduce a new framework of foundation, community and aided schools which would incorporate the GM sector.
"We shall bring in the necessary legislation as soon as possible," he said. "Meanwhile, the existing law remains in force, allowing schools to apply for GM status ... we shall consider each application on its individual merits. "
He already has 33 applications for GM status on his desk - nine of them are to build new GM schools.
At the same time as giving approval to Westoning Lower he turned down an application by Mount St Mary's Convent, an independent girls' school in Exeter, to opt into the state system.
Jan Marshall, head of Westoning Lower, was delighted by the approval. "We are very pleased and excited because we have been working very hard towards this."
She had received conflicting reports about whether Labour would approve the application and was unconcerned that the school may have to change its status to comply with new legislation.
"In education we have become very used to having to change. We as a school would be supportive of any change that helped raise standards. We went GM because we felt that was the best option.
"We didn't want to go GM for financial reasons and another change of status doesn't hold any threat to us. We have tried to keep out of the political arena as much as you can in education."
Mrs Latham said Labour could have exposed itself to the threat of judicial review if it had not treated decisions on GM status on their merits.
"I am pleased that it is actually applying the law as it stands now. That means schools can still go ahead and ballot."
Schools that have already opted out are unhappy at the prospect of being returned to local authority control. Paul Howson, head of Great Marlow Secondary, in Marlow, Buckinghamshire, said: "Once you have been able to work without the local authority you do not want to turn the clock back. If I was head of an LEA school I would still have to kowtow to them."
Barry Sindall, head of Colyton Grammar, in Colyford, Devon, said: "We would never want to go back to being under LEA control. We have thrived as a self-governing school both in terms of our placein the school league tables andin terms of staff morale."
Mike Mayers, head of Pingle Comprehensive School, in Swadlincote, Derbyshire, said: "We wanted to go GM not for any dogmatic reasons but because we believed it would be better for the children and the school. We value being able to focus on particular targets very quickly; the additional money was less important."
George Dixon School in Birmingham has received at least Pounds 600,000 extra from the Government since becoming grant-maintained in 1994. Julian Souter, deputy head, said it had raised standards dramatically and the percentage of pupils achieving A to C GCSE grades had more than doubled. "Labour's manifesto was incredibly vague. We are concerned that all our achievements will be rolled back," he said.