It happened around 8 a.m. at Farrell Elementary in the 8300 block of Castor Avenue.
The principal placed the school on lockdown after a student brought a gun into the building.
The student is in custody and the firearm has been recovered, police say.
The building was deemed safe and the lockdown was lifted at about 9:30 a.m., officials said.
RELATED: Check the 6abc Neighborhood Safety Tracker