Cal. VEH § 546

A “school pupil activity bus” is any motor vehicle, other than a schoolbus, operated by a common carrier, or by and under the exclusive jurisdiction of a publicly owned or operated transit system, or by a passenger charter-party carrier, used under a contractual agreement between a school and carrier to transport school pupils at or below the 12th-grade level to or from a public or private school activity, or used to transport pupils to or from residential schools, when the pupils are received and discharged at off-highway locations where a parent or adult designated by the parent is present to accept the pupil or place the pupil on the bus. As used in this section, common carrier, publicly owned or operated transit system, and passenger charter-party carrier refer to carriers in business for the principal purpose of transporting members of the public on a commercial basis. This section shall not apply to a motor vehicle operated by a carrier licensed by the Interstate Commerce Commission that is transporting pupils on a school activity trip entering or returning to the state from another state or country.

The driver of a school pupil activity bus shall be subject to the regulations adopted by the California Highway Patrol governing schoolbus drivers, except that the regulations shall not require drivers to duplicate training or schooling that they have otherwise received which is equivalent to that required pursuant to the regulations, and the regulations shall not require drivers to take training in first aid. However, a valid certificate to drive a school pupil activity bus shall not entitle the bearer to drive a schoolbus.