What constitutes unlawful passing of a school bus?

Unlawful passing of a school bus is defined by specific actions that endanger children who are getting on or off the bus. When a school bus stops and activates its stop sign, it indicates that children may be entering or exiting the vehicle, which poses a risk for inattentive drivers.

Passing when the stop sign is extended is considered unlawful because it directly violates the safety measure in place to protect students. The extended stop sign is a clear signal to drivers that they must come to a complete stop and remain stopped until the sign is retracted, as children may be in the vicinity.

Additionally, passing while children are boarding or exiting the bus is equally dangerous and unlawful. This situation can create a hazardous environment where a child might suddenly step into the road, potentially leading to tragic accidents.

Given that both actions described—infringing upon the bus's stop sign and ignoring the presence of children around the bus—are illegal in Iowa, the correct choice identifies that both constitute unlawful passing of a school bus, reinforcing the critical need for drivers to adhere to these safety laws to protect children.