I'm so sorry you have to go through this.
When I was in my first year, the girls in our flat "bonded" by picking on one of the boys we lived with.. They made him miserable! There was this really loud cockney one who would make horrible remarks about him both behind his back and to his face, it was almost as though they were united in their contempt for the guy. He was nice, actually. Just quiet, and a very easy target.
He moved out just before christmas, and guess what happened then? They had nothing to talk about so they started bitching about each other, and the rows in our flat were unbelieveable.
It probably won't make you feel any better hun but I'm convinced this is a similar occurance at your place, they need to feel united in their hatred of somebody and you were their easiest target.
Pretty much the same thing happened at my friends house last year (which is why she spent most of the year at mine) and it's just petty bullying. I bet they feel much better about themselves by putting you down, and sounds like they sure think they're better than you.
If I were you I'd ask about moving to a different flat or something.. Your first experience of uni should not be like this and they will only have each other to pick on in your absence.
Hope this helps.