Hello NatalieMK, I'm sorry that you are going through such a hard time at school right now. Honestly, I agree with everything Ada has said, and have very little to add to her analysis but I still would like to try and help you through this time in your life.
To start off with, I do not think you are a sadist- or at least not as much of one as you fear you are. If you only feel sadistic about this one girl who has wronged you many small times and want to "repay" her with sadistic acts, then it is much more a revenge fantasy. Additionally, if you are being teased for being shy, awkward, etc. and you only want to harm your tormentors to repay them for their torment that should be another clue to the presence of a revenge fantasy. These sorts of fantasies are fine, and do not make you a "bad person" by any stretch of the imagination. Nor do they make you a sadist.
And, while I agree that you should not act on your revenge fantasies regardless of how much this girl and your other tormentors have wronged you, repressing yourself and these desires is not the right course of action to take. If you are a sadist (as in if you have these feelings toward those who have not tormented you, and would like to inflict pain on a possible partner or willing participant) then I recommend finding a willing masochist to play with, I certainly did
. And I understand that accepting desires, especially desires like these can be very difficult, but once you do it can transform your self-esteem, social skills, friendships, relationships and so much more for the better. People admire self confidence, and that stems from self-knowledge. You can never really be totally confident in your own abilities until you know yourself completely. So, embrace these desires as a part of your personality, don't repress them and hope that they go away.
Finally, I think you are on the wrong forum. I second the idea to go to the bullying forum to seek some advice from those more qualified to give it. It's a terrible thing that is happening to you, and I hope that those wiser than myself can help you to find a way through it. Bullying is never easy, but I am hopeful that with some advice and some guidance you will blossom into a lovely young woman, who is acutely self-aware, and far more confident than she ever thought possible. Best of luck to you in this difficult time in your life.
-MJH