A leaflet produced by PASS, a group representing parents of children in grant-maintained schools, says only the Conservatives plan to retain GM status if re-elected, then asks readers to vote to ensure the schools remain unchanged.
PASS, also known as the Grant Maintained Schools Parents' Association, came under fire for "brazenly" encouraging parents to vote Conservative.
Local Schools Information, a pro-local authority schools advisory body, also said heads and governors of GM schools could be breaking the law if they used pupils to distribute the newsletter.
PASS, which defines itself as a strictly non-political body, denied it was promoting one party and insisted it wanted parents to challenge all candidates over their views on opting out.
The PASS newsletter, called PassMark, is headlined "The vital choice you face about your child's future". It lists the achievements of grant- maintained schools, and claims they have failed only in getting all political parties to agree about them. "We are shortly to have a General Election and, sadly, the future of your child's school may be at stake".