A 15-year-old Diamond Ranch High School student was arrested Friday afternoon for allegedly bringing an unloaded revolver to school. The teen’s name wasn’t released because he is a minor.

Sgt. Erick Kim of the sheriff’s Walnut/Diamond Bar station said deputies found the gun in the student’s backpack while the boy was in class. Deputies found out later that the gun was reported stolen in 2008 from San Bernardino County.

It’s not known yet why the Pomona teen brought the weapon to campus which is located at 100 Diamond Ranch Drive in Pomona. The incident began at 12:31 p.m.

“The school called us and said they had information a student has a gun,” Kim said.

According to deputies, three students who became aware of the gun in the teen’s possession alerted school officials. The 15-year-old was in class when deputies arrived. They detained him and searched the classroom. The teen’s backpack was on the floor next to his desk. Inside the bag was the gun.

The teen was arrested on suspicion of possessing a firearm on school grounds and was taken to Los Padrinos Juvenile Hall in Downey.