Anaheim police Lt. Bob Dunn says three of the injured - the driver and two of the children - are in critical condition after the Thursday afternoon crash. The other nine students had minor injuries.
Television news reports show the Orange Unified School District bus tilted sideways and leaning against a tree on a roadside embankment next to Anaheim Hills Golf Course. Police say no other vehicles were involved in the crash.
The driver was trapped inside the bus, and firefighters removed him before he was taken to the hospital.
The California Highway Patrol will take over the investigation. There is no immediate word on the cause of the crash.