For the three years I actually took a school bus to school, I remember it being dependent on the driver. Most drivers would let us do normal "kids on bus" things: swap seats, pass notes, be loud and annoying at each other. Nothing outside the windows, no jumping over the seats. We never got warned, but if we got out of line, we had trouble waiting at the school for us the next time we got there.
One substitute driver we had about three times actually mandated no open windows, no talking, no standing, sit in the next available seat and stay in it, no music. We all told him how much he sucked as we left the bus (literally "dude, you suck" from about half of us as we exited). I think enough people complained we didn't get him again.
Keep in mind, there's one driver, and about 50 kids. And the driver has to have his/her back to those kids most of the time.
Your report might actually help that driver get assistance keeping kids in line that he/she is afraid of, rather than hurting the driver. Could benefit everyone.
Keep in mind - if the kids fully expect to be expelled for *something*, and their homelife is punishment when they DO follow the rules (or there are absolutely no consequences at home regardless), there's very little recourse for disciplin - no threats are worse than what they've already got.
I was about 16 when I realized that getting suspended was kind of awesome. Required vacation. Granted i never did, but still.