The girl was identified by the school as Leticia Ramirez.
Witnesses say the teen was running across the street at Perris Boulevard and Nuevo Road when she was struck by the sheriff's vehicle.
She was transported to an area hospital, where she died.
Investigators said the teen was not in a crosswalk when she was struck.
At the time of the collision, the sheriff's deputy was responding to a routine call for service within the city of Perris and did not have lights and sirens on, officials said.
The incident is under investigation by the California Highway Patrol.