What must a school bus driver do before leaving the driver's compartment with pupils on board?

A school bus driver must set the parking brake and remove the key before leaving the driver's compartment with pupils on board to ensure the safety and security of the students. Engaging the parking brake prevents the bus from rolling, which is crucial when there are children on board who may not fully understand the risks of being near a moving vehicle. Removing the key is essential to prevent unauthorized individuals from accessing the bus and potentially putting themselves or others in danger by moving the vehicle. This procedure is a standard safety protocol designed to protect the students and maintain order while the driver steps away from the driver's seat.

The other options, while important in different contexts, do not directly address the critical actions necessary for securing the bus and ensuring the safety of students as the driver needs to prioritize.