An investigation is to be launched by the Welsh government after a Catholic lobby group urged schools to back a campaign against same-sex marriage.
The Catholic Education Service (CES) wrote to its schools and invited them to support the online petition by the Coalition for Marriage.
Schools and teachers are forbidden from promoting a one-sided political argument.
CES denies breaking the law by allowing political campaigning in schools.
Education minister Leighton Andrews has asked his officials to investigate.
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