The accident occurred on the 11900 block of Athens Drive around 2:25 p.m.
Investigators say the bus was parked at Box Springs Elementary when it rolled backwards, struck the girl and pinned her under the wheels.
The victim was standing on the sidewalk and waiting for a ride home when the bus hit her.
The underage victim was rushed to the hospital for surgery. Her injuries aren't considered life-threatening.
The driver said she was activating a wheelchair lift at the back of the bus when it started rolling.
The California Highway Patrol is handling the ongoing investigation.
A crisis team was brought to the school campus Tuesday to help students cope with the accident.
CNS contributed to this report.