38. The driver of a vehicle used for the transportation of school children shall not leave the vehicle when there are school children on board, except in an emergency or to assist a handicapped school child who requires his assistance in getting into or out of the vehicle. In such instances, the driver shall turn off the engine, remove the switch key and apply the hand brake, except where the vehicle has a safety locking device which prevents any person other than the driver from setting the vehicle in motion.