Here's my take...
I am familiar with the bus situation in our area. Here in our little Connecticut town, every bus has a camera. (Notice I said "has" a camera-they are on maybe once a month. That's helpful for policing the buses...Not.) No driver is capable of driving, stopping at the right spots, keeping an eye on the roads, and watching dozens of 6-12th graders all at the same time. Unfortunately, seating is very tight, which causes several arguments. There are nasty people, people with, shall we say, colorful language, and wierdos. (And by that I mean people who have gotten their hair dyed rainbow and where odd outfits by 6th grade. Yikes!!) Luckily, there are several responsible people. Most of the high schoolers on the buses cause little trouble. Many let younger kids sit with them, as they do homework or listen to music. The problem is not sixth graders. They just need to be a little more assertive. The issue is with some rowdy 7th and 8th graders who are bossy to younger kids because they can be and loud because they can get away with it.
The bus driver doesn't know anyone's names, and relies on a little list of rules that doesn't get enforced. (no food, no talking unless the person is immeadiately next to you, etc.) This adds to the problem.
It all comes down to the fact that there are too many people on the bus today. n our town, everyone double up in their seats, and in some cases triple. ( And those bus seats aren't particularly roomy!

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Your best bet is to find out hte kids name. Here, people can be expelled from the bus if the situation warrants, and this certainly does. Definitely mention that the kid did not listen, and was told REPEATEDLY to stop. I hope that his disgusting behavior stops.