My bus driver would stop and lecture us about this. His reasoning was that in case of an accident, evacuation is difficult if all the kids are buckled in. So if there was a fire (not sure how likely that is), we'd be worse off with seat belts. Now that I'm older, I'm pretty sure he wasn't speaking from a point of deep experience or research, but he was very passionate about the subject, enough to drop us off late several times due to such lectures.