It is believed the attack happened after the girl passed out after drinking at a party.
One of the rapists later told police: "She can't complain. Women must obey men."
The shocking assault happened in November but was only discovered this week by a teacher at a school in Ostend, Belgium.
A police probe was launched after a 14-year-old boy at the Ostend Technical Institute bragged about a photo of himself dressed in military fatigues and holding a sub-machine gun.