When can school crossing patrols indicate that pedestrians should not cross?

The correct response is when the school crossing patrol is holding their arm up. This action serves as a clear and standardized signal to indicate to pedestrians that they should not cross the road at that moment. In conjunction with training, the raised arm signifies that it is not safe for pedestrians to proceed. This visual cue is critical for maintaining the safety of both pedestrians and motorists, especially in high-traffic areas where visibility and awareness are essential.

The other options do not effectively communicate the same level of authority or clarity. For instance, signaling for vehicles to stop does not directly imply that pedestrians should not cross, as there may be confusion regarding which party has the right of way. Similarly, having no visible traffic does not necessitate a stop for pedestrians, and waving at pedestrians lacks the specific intention and recognition associated with arm signals, potentially leading to ambiguity about safety. These distinctions reinforce the importance of the recognized signal of the arm being held up as the definitive action indicating that pedestrians should wait.