At least 44 schools in England and Wales have sex and relationship policies which resemble Section 28, a decade after it was repealed.
An investigation by the British Humanist Association (BHA) found the establishments' policies either replicated Section 28 or were unhelpfully vague on what would be allowed to be taught in class.
"All of the schools identified must urgently review their sex and relationships education policies to ensure that all pupils, whatever their sexual orientation or gender identity, are treated with equal respect and understanding," BHA head of public affairs Pavan Dhaliwal said.
Section 28 was a much-hated piece of legislation passed by the Thatcher government outlawing the "promotion" of homosexuality in schools. It came to be seen as a symbol of homophobia in government until its repeal by Labour in 2003.
But concerns have been raised about a below-the-radar maintenance of Section 28 by some schools after it emerged that Colston Girls' School in Bristol's sex and relationships education policy stated: "The governing body will not permit the promotion of homosexuality."
The school immediately removed the section in question after the BHA raised the issue, but the discovery prompted the organisation to take a more extensive look at individual schools' policies.
It found 19 policies where it believes there was something clearly offensive and a further 22 that still seem to imply that section 28 might some way be in force. Another three were otherwise vague.
Labour shadow education secretary Stephen Twigg commented: "Michael Gove must intervene to ensure that all schools obey their duties under the Equality Act. It seems that some schools, perhaps not knowingly, are still using guidance from before Labour’s repeal of Section 28."
The institutions included the Grace Academy chain based in Coventry, Darlaston and Solihull, which says it does not "permit the promotion of homosexuality" and the Tasker Milward VC School in Pembrokeshire, which says it will "not intentionally promote homosexuality or publish material which actively promotes homosexuality".