Janis was also a Mean Girl

She was getting revenge that she rightfully deserved. It's all Reginas fault for being a bad friend to her.

Wasn't that kind of the point? All girls at that age will do at least a few things that would qualify them as a "mean girl". Even ones that are normally really nice. Even worse, they'll do it to another girl whom they consider a friend.

When Tina Fey asked the girls in the gym to raise their hands if they ever said something about someone that wasn't true, everyone raised their hand. And then they all took a turn doing the apology fall.