1. I don't know about the others, but isn't the rabbit Carl?
2. Well, Brain is almost unrealistically brilliant and clever, so he probably figured out a way to install his supply despenser into his desk... but somehow, I doubt he received any kind of permission, because surely his superiors would have said no.
3. Given the timing of Mr. Haney's entrance, they all probably would have been serving detention, there would have been a LOT of phonecalls to parents, and who knows what else?
4. Buster definitely would have gotten into trouble... he probably would have been given detention, or maybe even suspended.
5. They were all caught up in the heat of the moment (not to mention the heat in general), a lot of them probably didn't even realize what they were doing until after all was said and done... remember in the end, Binky even asks what started the fight, to which Buster and Brain both shrug and say, "I don't know."