It's a good question.
We did a little digging and found an old 2010 report in Today that described it as such: "Because modern school buses are already remarkably safe, and because seat belts don't work the same way in buses as they do cars, research shows." The report goes on to cite a figure from the National Safety Council that school buses are roughly 40 times safer than the family car.
School buses are only 'safe' until a child is seriously injured.
As you continue to read the report, though, you see that most of the rationale for why school buses don't have seat belts is cost. Granted, cost can be a legitimate concern, and can be a significant reason as to why school buses lack them, but cost is often a factor when it comes to the underlying reason why safety measures aren't taken in the first place.
No seat belt can mean not staying put.
A rollover crash, which results in one or more children being thrown from the bus, is a prime example of an injury that could be prevented with the use of seat belts. The Today report describes just such a case, in which two students lost their lives in Texas in 2006 when their bus rolled over. The case resulted in new state law requiring seat belts on some buses. Fast-forward to 2017 and you'll find lawmakers in Tennessee looking to do something similar.
Tennessee lawmakers have proposed three new bills in 2017.
State lawmaker Joanne Favors' bill would require seat belts on Tennessee school buses no later than July 1, 2023; U.S. Congressman Steve Cohen (of Tennessee) has filed a similar bill that would require seat belts nationally; and state lawmaker David Hawk's bill would impose additional safety requirements on school bus drivers.
This is a step in the right direction.