Muslim parents at two single-sex secondaries are campaigning to prevent the schools merging to form a mixed comprehensive, claiming that this would adversely affect girls' education.
The parents also believe that merging Batley boys' and girls' high schools into a single secondary could lead to religious radicalisation within the community.
Kirklees council has proposed that the two West Yorkshire schools should be replaced with one co-educational comprehensive of 1,650 pupils, under the Building Schools for the Future programme.
Parents were consulted, but initial talks took place during Ramadan, so there was only limited feedback from the schools' parents - 90 per cent of whom are Muslim.
Since then, more than 9,000 parents and community members have signed a petition asking that the schools are kept separate. The signatories include many of the parents of pupils at both schools, as well as feeder primaries.
Mohammed Pandor, one of the campaign organisers, said: "We believe that girls and boys are best educated separately. Girls do better in single-sex schools."
He said he believes girls and boys mature at different rates and learn in different ways. Similarly, subject choices are more likely to be polarised - with boys opting for science subjects and girls choosing the arts - in mixed schools.
"Our parents don't want to send their children to a Muslim school," he said.
"There is a private Muslim girls' school in the area, but it's very small. They want state education. This is taking away parents' choice."
The local Muslim community is conservative: men and women sit separately at weddings and community centre functions.
"So at the age when testosterone is flowing, we believe mixed classes are just asking for trouble," Mr Pandor said.
"This flies completely in the face of democracy. When you go through democratic channels and they don't listen, that creates potential for radicalisation."
Pat Langham, former president of the Girls' Schools Association, questioned why Kirklees would want to alter a successful system.
"I regard this as more an education than a faith issue," she said. "While these parents wish for single-sex education on cultural grounds, they share my belief that actually it's in the best interests of both their sons and daughters to be educated separately."
The consultation will not conclude until January 28.
Councillor Jim Dodds, education spokesman for Kirklees, insisted that a similar consultation two years ago showed overwhelming parental support for a co-educational school.
"The desire for co-ed provision continues to have significant support," he said. "Senior officers within the education service continue to have frank and open discussions with community representatives to find a practical way forward."