I have no idea how old you are (guessing about 16?) but it's time to grow up. Don't let other people influence who you want to be.
"Fitting in" is not important, chances are that in a couple of years time you will have nothing to do with the people you are "friends" with atm.
Be yourself, that takes more strength than being a bully!
Stand up for what you believe in, even small things like for example you don't like a movie all your friends like - that's fine, they may give you crap for it but most people will respect the fact that you don't just agree to be "the same". (and you often find if you're the one that says she didn't like it all of a sudden other people will speak up too.
Treat people the way you want them to treat you and they will help you become a mature, happy person!
Life is to short to hang out with people that make you feel like trash!!!