"Agreed. Picking violins and violas only seems like it's favoring not smaller instruments, but instruments considered more "highbrow," which is just silly."
My guess isn't that they favor violins, just that that's the instrument they chose for rule of thumb, and I'd bet that any instrument that's smaller and lighter than a violin would be allowed on the bus (except oboes, which as we all know shouldn't be allowed anywhere near children who might be tempted to play them). Besides, I've seen cell phones larger than a piccolo case, so you could avoid that argument by putting it in your backpack.
"School busses normally have cargo space underneath -- it would be a pain for the driver, but the "it won't fit" argument doesn't work in light of that."
Maybe your busses are different from the ones I've seen, but I've never seen a standard yellow school bus with underside cargo space, unless you mean the space under the seats.
"To me, the control requirement makes sense, but the lap requirement does not, because DSS could place the trombone upright on the floor, held between his knees, and it would present no more of a safety issue that if it was on his lap, because he can control it. I also doubt that the students are all holding their backpacks on their lap; I know when I was in school we threw them on the floor. If need be though, he could put the trombone on his lap, it would just be stick up about a foot above his head."
This is a tough call, because the issue of control is more than just keeping it under control under normal circumstances. I agree he could hold a trombone case in his lap for a normal ride, but I seriously doubt his ability to control it if the bus went off the road and it could seriously injure him or others in that case (sorry about that pun) where a smaller or lighter instrument would be easier to hang on to. Moreover, in an accident, a four foot rigid case is much more likely to block egress than a backpack which is generally soft and rarely more than two feet to a side, so again I see the reason why there's a restriction in the first place. The best solution would be a cage (which someone mentioned earlier as bins in the back of the bus) or bus seat legs that are designed to allow the case to slide under the seat for travel.