The school wasn't placed on lockdown, as may have been reported, but students were held in their classrooms for a short time as deputies searched for the 15-year-old, according to a news release from Ryan Hightower, spokesman for the Pomona Unified School District.
Someone at the school contacted deputies to report they had seen the unidentified student with the firearm, said sheriff's Sgt. Erik Kim.
Deputies, with help from school administrators, located the student in a classroom and reportedly found the unloaded weapon in the teen's backpack near the student. Investigators said they later learned the firearm had been reported stolen in 2008.
It was not immediately clear why the boy had the firearm.
He was handcuffed and led away and was being held at Los Padrines Juvenile Hall in Downey on suspicion of being a minor in possession of a firearm, possession of a stolen firearm and possession of a firearm on school grounds, according to a news release.