How do I deal with friends?

Ok so my "friends" are and are always using me as the target of their insults and jokes for no reason, I am the least flawed of them all and I barely react to it but they still decide to target me, I know it isn't my reaction because they once decided to turn on another one of our friends and he started crying even though they didn't insult him and they just decided to continue targetting me. They are pretty stupid also, and mindlessly follow each other and believe I am the hole and that they are the "good guys" even though I never insult them or do anything harmful towards them. So I just decided I didn't want to hang out with them anymore so I told them to leave me alone and just sat by myself and lunch but they just kept coming back and bugging me, even though I told them to leave me alone and I didn't want to hang out with them multiple times, by sitting with me so I just decided fine ill let them sit with me if they don't continue insulting me. Ok then they told me they were completely over what happened and that we should hang out during the summer so I said alright but now they are being stupid again and decided to go out without asking me or telling me that they are, they actually think I won't find out, well I did because I accidently stumbled on the information. I mean really, if they don't hang out with me then leave me alone and don't tell me you are over what happened and don't ask me to hang out with them during the summer. Now I have to go to the trouble of telling them to **** off and to let me sit by myself, why couldn't they just have left me alone god -__-. How do I go about leaving the group without having to explain myself, because if I suddenly decide to sit by myself again they are going to start asking questions and I don't want to start some immature drama about them not inviting me because I really don't care but I don't want to continue hanging out with hypocritical , whats the point they don't like me and I don't like them.