(a) Except as provided in subdivision (b), a schoolbus endorsement is valid only when the operator possesses or qualifies for a valid commercial driver’s license with a passenger endorsement and possesses a schoolbus driver’s certificate issued pursuant to Section 12517.
(b) A schoolbus endorsement is valid without a schoolbus driver’s certificate for an operator who is employed as a mechanic or a schoolbus driver-trainee if the schoolbus endorsement is restricted to operating a schoolbus when there is no pupil being transported.