Holy Cow! That is NOT acceptable! Children who behave like that really get on my nerves... That said, I am a mother of 6 children, four of which ride the Dodds school bus to school regularly. We are currently stationed in England... but as far as I know all Dodds bus systems are the same.
I would call the school first and make a formal complaint and at the same time ask for the school bus office number.. (Not sure if you've already done this) but the school bus office people will become your best friends... All you have to do is describe to them what the children look like, act like and what bus they ride on (Have your son there if you can)... and they will know exactly who you are talking about. They will also make phone calls to those parents and believe me... kids act better once their parents are involved... also, after three offenses they are kicked off the bus.
Don't be afraid of telling on them... that's what bullies like that thrive on -the kids that don't stand up for themselves... also if those kids know there's a parent involved they generally back off anyway.
I would definitely want to know if one of MY children were misbehaving... and as a matter of fact have had to deal with one of my own children (1st grader at the time) for rude behavior on buses in the past. He is really good now! (It seems lots of extra chores after school isn't what he had on his adgenda when he came home! hahaha!)
Also the key is to really get to know the people at the bus office, call them EVERY day with complaints if you have to. I promise it will be stopped.
And if it doesn't work... we're moving there this summer and my kids will go to the elementary school and I'll help you fight it next fall.. No way will my kids feel afraid to travel to a NEW school, I mean they already have to leave home... why make them even more insecure..? besides if the system isn't working properly then how am I to know if my kids are the ones causing problems?