Actually, all buses are exempt, not just school ones. My understanding is that the booster seats are to make air bags and seatbelts fit and work properly. Buses have neither of those, so booster seats wouldn't do any good. And technically, anyone under a certain height (minor or not) should use a booster seat when riding or driving in trucks and automobiles.

Also, i suspect (though don't know), that bus drivers must get special license endorsements and are thus under the law considered superior drivers whose riders need less protection.

You, sir, are making too much sense. It's not tolerated in certain circles.