As mentioned, in a lot of suburban areas in the US where school buses are common there are no crosswalks or traffic lights (or sidewalks or physical bus stops, for that matter). Most of the time there isn't so much traffic that stopping all of it is a huge burden.

Also, there's generally an exception for divided highways - if the road has a physical median or barrier, the oncoming traffic doesn't have to stop. I assume the bus route accounts for this and drops kids off on the correct side of the road.