I've dealt with kids like that when I went to school before I graduated. The best thing to do is do nothing, leave them alone. People like that will often go after the ones who are the nicest among the students. I know, I was victimized by many different groups of kids, but when it came to girls they always seem to be in a group.
The best way to cause this group to become destabilize the group is to observe the leader of this group, see how she acts, and how she is towards her friends, but observe them for a few months. After a while, if she insults you insult her back with a comment that will shatter her psyche for a little while. The leader will leave you alone and actually go after her friends.
That is how it worked for me. The experience I had was with a girl on the schoolbus who would insult me all the time. I just ignored her and observed how she acted on the bus. She would sit upside-down, lean fully across the seats, and jump in her friends laps. Towards the end of the school year she just pissed me off enough I told someone that I had a prediction for her.
I said "I predict that in the next ten years this girl will be a stripper who's found dead behind a dumpster somewhere."
Well, the person I told this to told the girl what I said and went to the bus driver saying I threatened her. Since I sat up front and the driver saw everything he didn't do anything, and I told the girl "I don't threaten, I'm just predicting what will happen to you in the next ten years." She left me alone and went after her friends and lost a few of them over the summer break.
She started to insult me again and I just told her to remember my prediction and she left me alone ever since.
But usually walk away, the only reason I said that was because I just snapped at her. I do not recommend doing that unless you have no other option.
