Which piece of equipment is NOT required on a school bus?

The correct choice highlights that a public address (PA) system is not a required piece of equipment on a school bus. In the context of pupil activity buses, the focus is primarily on safety and readiness for emergencies rather than communication.

While having a PA system can be beneficial for communication to students or during emergencies, it is not mandated by regulations. In contrast, items such as a two-way radio, fire extinguisher, and first aid kit are essential for ensuring the safety of passengers and drivers. The two-way radio facilitates communication with other vehicles or authorities, the fire extinguisher is crucial for fire safety, and the first aid kit provides necessary supplies in case of medical emergencies. Therefore, understanding the necessity of required safety equipment over supplementary communication tools clarifies why a PA system is optional rather than obligatory.