Cal. Education Code § 39830

A schoolbus is any motor vehicle designed, used, or maintained for the transportation of a school pupil at or below the grade 12 level to or from a public or private school or to or from public or private school activities, except the following: (a) A motor vehicle of any type carrying only members of the household of its owner. (b) A motortruck transporting pupils who are seated only in the passenger compartment, and a passenger vehicle designed for and when actually carrying not more than 10 persons, including the driver, except any vehicle or truck transporting two or more pupils who use wheelchairs. (c) A motor vehicle operated by a common carrier, or by and under exclusive jurisdiction of a publicly owned or operated transit system, only during the time it is on a scheduled run and is available to the general public or on a run scheduled in response to a request from a pupil who uses a wheelchair, or from a parent of the pupil, for transportation to or from nonschool activities. However, the motor vehicle is designed for and actually carries not more than 16 persons and the driver, is available to eligible persons of the general public, and the school does not provide the requested transportation service. (d) A school pupil activity bus as defined in Section 39830.1. (e) A motor vehicle operated by a carrier licensed by the Interstate Commerce Commission that is transporting pupils on a school activity entering or returning to the state from another state or country. (f) A state-owned motor vehicle being operated by a state employee upon the driveways, paths, parking facilities, or grounds specified in Section 21113 of the Vehicle Code that are under the control of a state hospital under the jurisdiction of the State Department of Developmental Services where the posted speed limit is not more than 20 miles per hour. The motor vehicle may also be operated for a distance of not more than one-quarter mile upon a public street or highway that runs through the grounds of a state hospital under the jurisdiction of the State Department of Developmental Services, if the posted speed limit on the public street or highway is not more than 25 miles per hour and if all traffic is regulated by posted stop signs or official traffic control signals at the points of entry and exit by the motor vehicle.