That is indeed the argument. It is reviewed regularly. I will leave it up to the safety engineers to make the call. The soccer moms will always opt for what would have been desirable in the last accident, but that's not a good way to make policy.
Busses are considered inherently safe because of their size, weight, and large crumple zones. A head-on collision with a tractor-trailer or getting T-boned by a train or sailing off a mountain cliff will be bad in any vehicle, with or without seatbelts, but busses tend to be forgiving in most accidents. Getting out quickly in the event of fire is important.
Unfortunately it doesn’t take a a collision with a tractor trailer or getting t boned to kill people in a school bus. It just takes an accident, because the passengers aren’t restrained.