I've asked the same thing. It's the law that they must wear them in the car but throw fifty of them together in a big steel can and don't worry about it?
It doesn't make sense.
The buses here do have seat belts but they don't make the kids wear them so none of them do.
My nephew's bus was involved in an accident years ago. It wasn't too bad but the kids were tossed around and a few ended up in the hospital.