Carpundit has been advocating seatbelts for schoolbuses, and I agree with him. I am not convinced by the arguments I have read that kids shouldn't be belted in. In any collision with a heavy or stationary object, the kids will fly around and knock into one another, or be ejected, as recently happened when a school bus collided with a garbage truck, killing a student and injuring many.
A major benefit to seatbelts: kids that are belted in will be less likely to engage in fighting or other disruptive behavior, since their mobility will be limited.
There are a few arguments that I have heard against seatbelts in buses that make any real sense. The first is that kids that are belted in will have a harder time escaping the bus, should it be on fire or under water. But, as Carpundit points out, these types of accidents are extremely rare.
I have read that when seatbelts are installed in buses, kids won't use them. But this is an enforcement issue. If kids are taught from an early age to buckle up, and bus drivers do a check to make sure that kids are wearing belts, the usage rate will improve. Eventually, it will be a habit.
Another argument is cost. Buses are not cheap to begin with, coming in at around $50,000 for a 72 passenger bus, and they are in service for around 15 years. If seatbelts cost $25 per seat, that is $1800, or a whopping 3% increase in cost. Amortized over 15 years, (without interest), we are looking at all of $120/year/bus for seatbelts.
A more subtle cost argument is that the money would be more effectively spent on student and driver training than seatbelts, that there would be better value in accident avoidance. The problem here is that there may not be enough statistical data out there to draw a solid conclusion. On the other hand, we know from first principles that seatbelts save lives.
There is an in depth article on this subject here.