My comment would just be reiterating the first two responses, but I will add that the stress caused by your "friends" is going to negatively effect your relationship with your boyfriend who, unlike these friends of yours, sounds like he actually cares about you . If you won't stand up to them for yourself, at least do it for your relationship.
I too would say that you have to work on the impression you give others about yourself. If you laugh off all the jokes people make about you, how do you expect them to take you seriously. Moreover how can you allow someone to throw things at you for no fault of yours, and then be friends with them again. I think its time you took yourself seriously and maintain your dignity, people who do these things to you cannot be called friends by any means.