Can you give me advice on what to do about my friendship troubles?

as of recent my friends at school have all began to be , honestly just plain nasty and mean to me. we are all 15/16 year olds, although they are quite immature well most of them anyway, in fact to be honest one in particular (Chris) is so bad that he actually ranks , and i mean ranks who he likes better in some kind of freakish list that he tells people occaissionally ( also everyone considers him the leader of our little group, even though he is the biggest back stabber i know, because of him, my mum's even been crying from fear of me having to be put on anti depressants, he's been fuking around with my head alot over the past few months) here are some of the things he has been doing to me, to get me upset he makes me think im going to get to come out with him and the others on the weekends, then he'll make up and excuse to try and make me feel bad. he's been turning people against me and as today has started running away at lunch with everyone of my other so called friends . all except one, a boy who i've known for almost a year, a boy who has been friends with the main problem since primary 1 ( this is how i know that he wont be able to keep this up for long, he is pretty much known for always being the one to follow (chris) and yet he stayed for me ,, he told me he'd always be my friend no matter what chris done. im scared that im going to lose him now, now moving onto another one (Dean) not even two months ago he was the one chris wanted rid of, he couldnt stand the boy he was always saying how much he hated him, nd wanted rid of him, which he was definately going to do. ./but he couldn't because i stood by dean and told chris he wasnt getting rid of him, basically i did one of the nicest things i've done for anyone. i was the only one who really cared. .and yet now he is another one who wants rid of me, to be completely honest, me and a few others have noticed that dean absolutely cant stand me when im with/speaking/getting on with mark (the boy who stuck by me) he actually gets jealous and tells people he's annoyed that i spoke to mark and not him, it's so childish. anyway mark wont be able to help me for very long against them, he'll be pushed into following them eventually, what do you think is the best thing for me to do( btw i have been worrying so much that i have nearly been taken to see psychologists , because i get really upset about losing friends , im mean literally sobbing uncontrollably.) one last thing i swear to god that if my mum ever gets upset because of that fat **** again, i will have no choice but to take a far more physical course of action towards him. i know i can beat all of them in a fight because A. none of them ever have been in a fight whereas i grow up in quite a bad area and B. they are all sneeky which means thay are nothing but cowards.