Do you know why?
Full shoulder belts are impractical in full sized school busses, and lap belts have proven to be as harmful as they are helpful. Smaller van-type busses are typically equipped with shoulder belts now, because they are smaller and occupants are more likely to be ejected from a seat in an impact.
Full size busses are designed to prevent this, and are quite safe. Seat belts have been used in full sized busses in the past, and they were found to be more likely to cause harm by either entrapping a passenger or causing direct damage to the hips, upper legs, lower spine, and abdominal organs. Seat belt entrapment has been considered as a possible factor in the deaths of passengers killed in busses that caught fire.
This is still very much a controversial topic. There are certainly instances where seatbelts may have prevented deaths and injuries, but by and large, school busses are one of the safest modes of transportation on the road in the United States. Numerous studies have been done on this subject. Almost any time a bus accident happens that gets significant coverage, people start demanding answers about "why didn't that bus have any seatbelts?", and the results are pretty much the same every time: because in the long run, they don't improve anything. Everything else about a school bus is designed to protect the occupants.
I recommend really looking into this subject yourself. It seems counterintuitive, but when you see the data for yourself, it makes sense.
The driver has a seatbelt because they are not sandwiched between seats explicitly designed to absorb energy and reduce injury. They have a steering column, dashboard, and windshield in front of them. There is no irony in their username, just ignorance on your part for speaking about a topic you know nothing about.