BBM. But the thing is, they are juveniles. They will be among other people. Which is why the juvenile justice system focuses on punishment and rehabilitation, because juveniles are released back into society. What I, and I think some other posters, are trying to point out is that if the punishment is excessive, then all that will happen is make these girls worse.
It's not an excuse to understand why someone bullies. Understanding is not excusing or mitigating. It helps tailor punishment and rehabilitation. If a child bullies because of an abusive home situation, that has to be handled differently than a child who bullies because of a mental health issue, or because the child is a sociopath.
Whether we like it or not, these children are going to grow up, become adults, and engage in society. How we handle this situation now can easily dictate how those later interactions will go. Many juveniles can be rehabilitated, even from terrible - I've seen it myself. Personally, I'd much rather punishment and therapy/rehabilitation that's tailored to the child, acknowledging what has caused the offense, so that when I have to meet a now-grown juvenile offender, I'm meeting him/her as a reformed adult, rather than someone ready to reoffend.
Bullies grow up. Some of the bullies I encountered as a kid have grown into good, caring adults. I have a hard time thinking of a child as worthless or irredeemable. I believe in the ability for any person, but particularly children who are still growing, to change, repent, and do good. Maybe that makes me a rose-colored glasses idealist, but I would hate for a child's life to be considered over and done, all for some bad choices made at the start of their lives.
ETA: I was bullied as a kid, around the same age as your daughter, FrayedKnot. I cried a lot, and it seemed like it would never get better - but it does. Keep on being a supportive mom and that will help so much.