My Daughter Is a Teen Bully – How Do I Make Her Stop?

I believe my 15 year old daughter is a bully to other kids in her school. Last year her homeroom teacher asked me to talk to her about the way she made some of the other less popular kids the constant butt of her jokes and gossiping and stuff. The teacher told me that there hadn’t been a single major incident and that while my daughter was popular and doing well in school, that she worried she might get into trouble as she progressed through high school if she didn’t change her ways. Her father left the family when she was 9. He seemed to encourage her aggression as a skill she’d need in life to get ahead and so he’d only laugh off or even approve of the aggressive behaviors (like hitting a playmate) that she would act out as a child. Now this seems to be the way she is and I can’t seem to make her change, no matter what I try to do. Talking to her seemed to have no real effect. She says that she’s not doing anything wrong and that those ‘losers’ at school who complain about what she does should worry more about losing a few pounds or getting a friend than complaining to the teacher about her (that’s how she talks about her classmates). I am trying to hammer home the golden rule, but when I see her on facebook at night I really worry that she isn’t behaving as she ought to. I love my daughter, but sometimes I cringe at the way she treats me and even her friends. I forced her (by withholding her allowance) to volunteer with me at the soup kitchen associated with our as a way to teach her some compassion for others, but the way she turned her nose up at the homeless people was so embarrassing to me and degrading to them, that I never made her go back. So, what can I do? The school isn’t doing anything and she doesn’t listen to what I have to say. How can I teach her to treat others a little better and to stop her bullying?