Which vehicles must stop when a school sign is present?

The correct understanding in this context is that all vehicles must stop when a school sign is present, particularly when it indicates the presence of a school zone with active pedestrian crossings. This rule is in place to ensure the safety of children who may be entering or exiting the school premises.

When you see a school sign, it is a warning to all drivers that children may be crossing the road or that there could be bus stops nearby where children are getting on or off. This is not limited to just the lane closest to the school or just certain vehicles; it applies to every lane on the roadway. Hence, the requirement to stop is universally applied regardless of the specific lanes or the presence of children at the moment, as the potential for children to be around is always a factor in these situations.

The other options suggest that only certain lanes or specific vehicles need to stop or that the presence of children is a determining factor. However, the law mandates that all vehicles must come to a stop in any lane when a school sign is present to ensure maximum safety for children who may be in the vicinity.