When we say school bus it’s an “everyone has a seat and is sat down” scenario.
With that established, I’m going to tell you that pretty much every EU country that runs school buses of that scenario does have seatbelts. We definitely do in the UK.
It clearly is possible. I’m not saying that the busses aren’t otherwise built for safety in every way like many other commenters are saying, and maybe they wouldn’t even provide extra benefit (which I doubt, but I see some commenters are citing studies), but it is definitely possible to add and realistic for kids to use seatbelts.