The lettered identification and traffic warning lights on the front and rear of school buses shall be covered with opaque detachable material when school buses are used for purposes other than to transport pupils on regular routes to and from school, on special trips to participate in contests of various kinds, or for supplementary education purposes as required by § 22.1-183 of the Code of Virginia. This does not apply when the bus is being used to transport elderly or mentally or physically handicapped persons.