Well...
In a nutshell, the reason school buses don’t have seatbelts is cost, the fact that school buses are already amazingly safe, and research to date has shown that adding seatbelts doesn’t actually make school buses definitively safer, and in some scenarios actually increases risk of injury to the child.
For alternate safety measures, the intentionally closely spaced seats (the bane of the knees of tall students) are extremely shock-absorbent and able to protect children effectively enough according to studies by the National Transportation Safety Board and National Academy of Sciences. Essentially, the seat design and spacing more or less functions as a “protective envelope” around the child. School buses are also some of the largest vehicles on the road and they aren’t typically driven very fast, further helping make them safe without seatbelts.
In a number of studies performed looking at just this issue by various transportation agencies, there is compelling evidence that the number of deaths wouldn’t change in any statistically significant way by adding seatbelts, and the number of injuries
may actually increase. (For instance, it’s thought that a short jolt forward into a heavily padded wall will in most cases result in fewer injuries than a strong jerk at the waist and the head smacking against said wall at an unfavourable angle, not to mention potential issues with longer evacuation times, particularly with primary aged kids, in the event of a fire, among other such scenarios.)
From a practical standpoint, there is also the difficulty of a bus driver making sure all the kids are wearing their belts in the first place and that they keep them on. The bus driver would also need to verify that the kids are wearing the seatbelts correctly at all times (incorrectly worn seatbelts pose a definite injury risk in an accident). Beyond this slowing down transportation times, it’s generally considered better that the bus driver is spending the majority of their time paying attention to the road instead.
In the end, as the National Highway Traffic Safety Administration noted with their research on the seatbelt issue going all the way back to 1987, all evidence points to that there is little, if any, benefit to including seatbelts in large school buses. The National Association for Pupil Transportation, the National School Transportation Association and the National Association of State Directors of Pupil Transportation Services all concur with this assessment based on their own research.
Instead, they all prefer to create “egg carton” safety envelopes that require the child to do nothing but stay in their seating area to keep them safe.
It seems to be working. Despite it being the number one way kids in the United States are transported to and from school, only about six students die per year in school bus crashes in the U.S. out of a total of about twenty-six million children transported throughout the school year. For comparison, a little under one thousand kids die every year in the United States while walking, biking, or being driven to or from school in a car.