Schools Out, a support group for gay and lesbian teachers and teenagers, said schools and local authorities were blocking websites with information about homosexuality, even though the sites were entirely appropriate for pupils.
Tony Fenwick, a co-chair of the group, is writing to schools and local authorities to warn them that they may be in breach of equality laws.
"This is an infringement of the Goods and Services Act because they're failing their equality duties by denying staff and students access to services," he said. "These filters are illegal."
Paul Patrick, a founding chair of Schools Out who died last month, had won a battle with his local authority to unblock access to such sites, Mr Fenwick said.
But many other schools and local authorities still had blanket bans on the sites.
A spokeswoman for Becta, the schools technology agency, said: "This is part of a learning process for schools in managing access to web content. It's such a fast-moving area that schools need to be aware of how some sites can be beneficial."
Facts for the deaf
For deaf pupils, finding out about the facts of life can be complicated. So Manchester University has launched a research project to make sex education more accessible.
A team is interviewing parents of deaf and hearing children and researching pupils' knowledge of the subject using an interactive computer game. Sarah Suter, who leads the study, said: "Children who do not have good sex education are more vulnerable to abuse because they may not recognise inappropriate behaviour, or that their boundaries are being crossed."
To take part in the study, contact firstname.lastname@example.org or call 0161 275 3391.