If It's Really, Really Bad, Try To Switch
Bad roommates are a rite of passage. One day, these horrible stories will be something you can laugh at with friends, but probably not right now. When a friend of mine had a roommate who was chucking Chinese throwing stars at him, it was terrible. Until he realized that his roommate had given him the ammunition he needed to make a switch. If your roommate is really terrible, as in the above example, the school should accommodate you in changing your living situation. If you can make a case that your roommate is a threat to your safety, there should be no discussion. Go directly to your R.A., report the situation, and demand a switch. If there is resistance, have your parents call the school.

It's Not My Safety So Much As My Sanity That's At Issue. Can I Still Switch?
Maybe. Sometimes, a school will take the position that learning to deal with difficult people is just part of growing up, and that while your situation is not ideal, you will grow from it. Though this argument has some validity, it probably won't comfort you when you're trying to study and you're roommate is throwing a party -- or vice versa. Again, you can go to your R.A. and see what can be done to accommodate you, but that person might offer you a shoulder to cry on and a pep talk about open communication about your needs and feelings and send you back to your room. What you can do to help yourself is to be on the lookout for someone who's in a similar situation -- or maybe you know that person already because you routinely commiserate about your bad roommates. If you can find yourself a potential living situation that will work better for you, go to your R.A. with your proposed solution. You might have to wait until the semester changes, but oftentimes, such a switch happens.

Talk To Your Roommate
When you have a bad roommate, you probably don't even want to see them, much less talk to them. But you should probably try this route first before investigating a change of living situation. Talking to people with whom you're not getting along is never easy, but if you can take a deep breath and tell your roommate what's bothering you about the living situation, there is always a chance that the two of you can compromise. Even if you don't become best friends, maybe your room will stop being the last place you'd like to go after classes.