Call his girlfriends parents....no seriously, you may need to get a lock for your door. Doesn't sound like there is much parental supervision going on over there. Where are your parents? They are really the only ones that can do anything about it.
Vid them...then tell him if he ever does it again, you'll post it on the web. You may also use this as leverage to have him make your bed, clean your room as well as other chores you may think are appropriate.
What is wrong with his bed? Anyways I would do the itchy white powder all over the sheets so when she goes home her parents will wonder what she has been up too. And also video tape them and use it for blackmale.
First off sweetie, I'm sorry you have to deal with that. Second, get in there and take control. If the door isn't locked, open it and yell for them to get out. If it does, bang on the door, windows, etc as it will help ruin the mood for them. Get NEW sheets... Actually I'm not sure I'd be able to sleep there period (again, so sorry!) Talk to your parents ASAP, although they don't seem to care. Call the non emergency line for your local police station, they can help. Above all, stay strong. You deserve so much more than that