There are a small amount of rules on public buses. I don't really know if they are the same for all bus companies, but on the CATA in East Lansing, especially on the routes that are right on campus.
1.) Exit out the back door. There are people coming in through the front door. So I don't care if you're in the seat right behind the bus driver, you walk to the back and go out there.
2.) Scoot all the way back. It is not uncommon for buses to literally leave people by the side of the road when the bus is full. The worst feeling in the world is getting left, knowing you need to be in class in ten minutes, and then looking in the window and seeing huge gaps in people. If everyone standing up would just scoot back, people wouldn't get left nearly as often.
Those are the official rules, but I think there are a couple more important rules that need to be addressed as well.
3.) Don't listen to your iPod so loud I can hear it. I listened to the entire new Beyonce album this way on a trip to the mall last year.
4.) Don't take up more than one seat. I'm not talking about people who are large enough to literally take two seats. That's fine and not your fault. I'm talking about girls who put their purse on the seat beside them and guys who sprawl out so there's only half of the other seat available.
I'm just saying, public transportation is bad enough. At least be polite.