Parents who fail to answer satisfactorily may find their child's application form relegated to the bottom of the pile.
Audenshaw High School in Tameside, Manchester, a boys' comprehensive which went grant-maintained in 1989, has around two applications for each of its 180 new admissions each year. Pupils are selected according to widely-used criteria, such as whether they have brothers at the school or live nearby. But parents are also asked to write on the application form why they want to send their son to a single-sex school.
"We're trying to find out if they have a commitment to single-sex education, " says headteacher Alan Crompton. "You're always trying to match the school to the child and the child to the school. It may be that many parents choose us because we give a good education, but we want to find out if they support the ethos of the school."
No one says they do not support single-sex education, says Mr Crompton, but around 30 leave the question blank - and get a lower priority.
Fairfield High School for Girls in nearby Droylsden, celebrating its 200th anniversary this year, wanted a similar policy, but was opposed by the Department for Education and Employment.
Lower school head Kathryn Quinn devised a questionnaire aimed at finding out how parents viewed segregated education. It asked them to look at statements, such as girls do better in maths classes without boys, and place them in order of priority. Parents were then awarded points for their answers. Those with the highest scores would be favoured in their applications.
"The idea was to determine which parent truly wanted to send their children here because it is girls-only," said Mrs Quinn. "But the DFEE told us it was inappropriate. We are still looking for an objective way of finding out which parents want to send their children here for the right reasons."