Probably they behave this way because they think it's smart and they're showing off to their friends. It also smacks of superiority. They feel that the bus driver is beneath them socially and so feel justified in behaving badly - after all he's only a Bus Driver!
Of course it's neither smart nor justified - just rude and immature, and as long as no one challenges them they will continue.
Giving advice is always tricky and can backfire, but I think something needs to be done, not only to relieve the pressure on the bus driver but also to ensure the young people themselves are made aware that what they are doing is not right. If no one acts now to show them the right way it's likely they will grow up as thoughtless and arrogant adults.
Don't challenge a group on your own, they'll put you down and make you look foolish. Get some support from like minded friends and plan. Before you go on the bus - get the spokesperson to start and say, 'Hey guys - the way you speak to the bus driver is not on, it's rude, uneccessary and it offends me to hear him spoken to like that - so just stop'
Of course you'll get called names, probably laughted at and derided etc, but now is the time for your supporters to say , 'Yeah that goes for me too' Now it's not just you. If you are challenged as to what you are going to do if they don't stop, you have to make sure whatever you do, you carry through. If you threaten to report them - you have to do it.
No one wants to be the telltale but sometimes you need to have adult intervention - they should act and sort it out if the rude behaviour contines.