I feel sorry for you. I was bullied a lot when I was in school. I just got used to it. If it is your friend's sister, ask your friend to tell her sister to leave you alone.
I found in most cases of the guys who bullied me, if you don't put up a struggle, or fight back, or let them know you even care in any way that they are there. Eventually they get bored bullying you, and find someone else to pick on.
I have never been bullied by a girl, so I don't know what to tell you there. They either liked me, or ignored me. But, it was ok, because I didn't want them around anyways.
The main thing is, just focus on being yourself. The Bullies go away as you get older. Especially when they find out they are not the greatest things to walk the planet in the future, and there is someone ten times tougher out there waiting to clean their clocks!