The Welsh government has written to all Catholic schools in Wales to remind them to give a “balanced perspective” when discussing same-sex marriage.
Education minister Leighton Andrews’ intervention comes after the Catholic Education Service (CES) invited schools to ask pupils to sign a petition opposing gay civic marriage laws.
Mr Andrews said the CES campaign was political.
CES denies breaking the law by allowing political campaigning in schools.
Both political arguments must be discussed in classes for the under 12s.
In his letter, Mr Andrews wrote the CES’s initial correspondence was “regrettable” because it did not make clear that only children over 16 should sign the petition.
“Whilst the petition itself is not directly related to a party political matter it does in my view relate to political matters generally as the petition is seeking to persuade people to lobby the Westminster government to prevent a change in the law,” he added.
He wrote he was writing to “remind” them of their legal obligations under the Education Act 1996.
“These provisions require that head teachers and governing bodies must not allow the pursuit of partisan political activities by any registered pupil who has not reached the age of 12 and must not allow the promotion of such views in any subject taught,” Mr Andrews added.
“I trust you will ensure that if your pupils have been made aware of this correspondence they will also be made aware of the converse view in order to give them a balanced perspective.”
BBC Wales reported last month Mr Andrews had launched an investigation in to the CES’s letter.
The CES has now issued new guidelines to its schools to remind them that children under the age of 16 cannot sign the online petition.