Final answer:
School buses should activate their upper alternating red signal lights before coming to a stop to load or unload passengers for safety reasons.
Step-by-step explanation:
When stopping a school bus to load or unload passengers, the correct procedure is to turn on your upper alternating red signal lights before you stop. This is a critical safety measure to alert other motorists that the bus is preparing to stop and that students may be entering or exiting the bus. Activating the red signal lights early enough ensures that drivers in the vicinity are aware of the upcoming stop and can react accordingly to prevent accidents.