Mich. Comp. Laws § 257.1831

Sec. 31. 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.