This is such a tough one. I was bullied by one girl at school so bad I ended up leaving and being home schooled.
She spread rumors, abused me in front of people, once she came up to me and punched me in the face literally for no reason, then ran off and laughed.
I tried ignoring her, then sticking up for myself, my mum got the school involved and while they were supportive.... There's not much they can do.
Girls bullying is a lot worse and you can't see it... So it's harder to stop and even harder to escape. I believe unless the parent of the bully is onboard, it's hard to stop them, unless the suddenly grow up.
I honestly don't know what I'll do if my dd is bullied and I've had such negative experiences myself. It scares me already and she's only 2!