I would blame the school.
Unless I work there. Then it’s the parents’ fault.
I think supervision on the bus would have prevented the issue to some extent. But on the other hand, it’s not that easy to manage a whole bus of children. They are more like sheep than we are and a few kids can make the whole bus go crazy. Unfortunately, the lead sheep is never a lemele. That’s just how children are. It’s our job to train them out of this behavior and teach them to be self-critical by the time they reach adulthood. But you don’t expect a 3 month old child to start walking and you shouldn’t expect bus-age children to not act like sheep.