Cal. Code Regs. tit. 13 § 1290
Bumpers on Type 1 school buses constructed after January 1, 1950, and on Type 2 school buses constructed on and after July 1, 1970, shall be installed front and rear and shall be attached directly to the chassis frame or other structural members of sufficient strength. Bumpers shall be strong enough to permit the bus to push a vehicle of equal gross loaded weight or be pushed without permanent distortion of bumper, chassis, or body. Rear bumpers of Type 1 school buses shall be designed to prevent anyone from getting a toehold and hitching a ride. A rear bumper is not required when a vehicle is equipped on the rear with a wheelchair loading device that, when retracted, meets or exceeds the protection provided by the original bumper.
Cal. Code Regs. Tit. 13, § 1290