A gay student at a religious high school in Italy was banned from class over his sexual orientation, his parents have claimed, triggering a furore across the Mediterranean country, which is already debating same-sex marriage.

The 16-year-old was ordered out of his classroom and placed to study alone in a hallway by the headmaster due to a photo posted on social media depicting him with another boy. As his parents became aware of the disciplinary measure, they reported it to police, claiming it was discriminatory.

"My son is being discriminated against because he is gay," the mother told Il Fatto Quotidiano newspaper. "His grades are good and teachers are happy with him."

The catholic institute's headmaster claimed the ban was issued for the well-being of the student, who is followed by social services, and his classmates, following the clamour that erupted at school after the incriminated photo emerged.

"This has nothing to do with discrimination, Christians do not discriminate, we accept everyone," Adriano Corrioni, the head of the Ecfop institute in Monza, north of Milan told L'Espresso magazine. "I thought it was right to remove the student to avoid that issues which need to be addressed by his family and social services be talked about in class".

Corrioni claimed the photo posted on Instagram was "akin to child phonography" but the mother of the boy disputed that, saying it was taken during summer holidays and simply depicted her son and a friend bare-chested.

The incident sparked furious reactions across the peninsula. LGBT rights group Arcigay said the school's decision was unacceptable and urged the government to take action.

Vincenzo Spadafora, the head of a public authority for children and teenagers' rights, said he was left "speechless" by the events and vowed severe measures against the school if the discriminatory nature of the ban was confirmed.

The controversy comes as the country is engaged in a heated debate over same-sex marriages, after the government pledged to introduce a law on civil unions. Italy is currently the only country in Western Europe where no form of gay marriage is recognised.