When a school bus is parked and left unattended, what should always be set?

When a school bus is parked and left unattended, it is essential to set the parking brake. This action ensures that the bus remains stationary and does not roll or move unexpectedly, which is crucial for safety, especially considering that school buses carry children. The parking brake is specifically designed to hold the vehicle in place when it is not in use, thus preventing potential accidents that could occur if the bus were to roll away.

While options like the ignition, headlights, and emergency lights are important for various driving conditions and scenarios, they do not provide the necessary security against movement when the vehicle is stationary. The ignition should be turned off to ensure the vehicle cannot be started inadvertently, but it does not keep the bus from rolling. Headlights and emergency lights are used for visibility and signaling, but they do not prevent the bus from moving either. Therefore, properly setting the parking brake is the definitive safety measure when a school bus is parked and unattended.