Mean Girl Syndrome is what teachers refer to when a consistently sweet little girl makes a catty comment or shows her claws in some way. It's not referring to a child who is always mean, just the ones that oh, so occasionally show that side of them that is usually repressed.
I'm going to step out on a limb and say I'm not usually mean. But occasionally, I revert back to a 9 year old little girl and very much want to make a catty comment.
When that happens, this is the conversation I have with myself:
Mean Girl: "Ooo, I have something good to say to put her in her place!"
Adult me: "No, mean girl! We've learned to be kind. You don't get a say! Shhhhhh!"