A school bus may be equipped with a flashing, oscillating, or rotating light mounted on the roof of the bus approximately 6 feet from the rear of the vehicle which displays a white light to the front, side, and rear of the bus. If a school bus is equipped with such a light, there is no requirement that a driver use it. The light shall not be actuated by the driver unless 1 or more of the following conditions exist:

    (a) Inclement weather such as fog, rain, or snow.

    (b) When passengers are boarding or being discharged.

    (c) From 1/2 hour after sunset until 1/2 hour before sunrise. (d) Where conditions hinder the visibility of the school bus.