What action is imperative before any bus moves with pupils onboard?

Before any bus moves with pupils onboard, it is imperative that all pupils are seated. This requirement is crucial for safety reasons. When students are seated, they are less likely to be injured during transit, especially in the event of sudden stops or accidents. Properly seated students help the driver maintain focus on the road as there are fewer distractions and movements within the bus.

In addition, having all pupils seated allows the driver to ensure that the bus is ready to move safely, as it provides the opportunity to conduct a final check of the bus's interior and confirm that all students are secure. Other options fail to prioritize student safety and could lead to dangerous situations on the road.