DD is 8, year 3, In her class is a mini Regina, complete with two "handmaidens". It really is straight out of Mean Girls, mini Regina is very intelligent and controlling (I know her mother quite well, the girl truly rules the roost at home) and dominates many of the other girls in the class.
The school as a whole is pretty good on stamping out bullying and the class have done lessons on not bullying and what to do if you are being bullied, but mean girl behaviour isn't excatly the same thing. It's not physical violence or name calling, it's so much more clever and insidious than that and as such harder to pin down or describe.
I know mini Regina bothers DD sometimes, I have tried to explain to her about girls and cliques but it's not an easy thing to define. I was thinking of watching Mean Girls with her, but I rewatched it again by myself and had clearly forgotten how many sexual references it contained, it's not suitable for an 8year old. What I'm looking for is suggestions for something similar but sutable for a younger age group.
Personally I would avoid sexist ways of explaining it. Boys and girls can be unkind. Boys and girls can feel vulnerable and sometimes they can both try to manipulate. I don't think it's helpful to make this sort of behaviour a 'girl' thing. Give her the understanding so she can tackle it anywhere wether it be from a female or a male. Don't perpetuate the idea that women are inherently nastier.