Perhaps it's because a school bus has the same passengers every day, who are all going to the same location every day, where there are few outsiders? Unlike a public bus which may have some passengers who take the same bus every day, but there are others who do not. And they are not all going to spend hours indoors together after they get off the bus. They are going to be exposed to different people, and may be riding home with a different mix of people later in the day.
The children are likely to live in the same neighborhood, so may even play together outside of school hours. So the children going to and from school are in a smaller "bubble" than people going to and from the workplace or other places.