Not all, but a lot of girls are bitches in the childish sense all the way into college. Now @Robert Lindsay was suggesting that boys had a monopoly on mean. Going into puberty, boys do have more testosterone, but they aren’t more evil.
What do mean girls hate? They viciously pick on weak boys/men and/or ones that feel have hurt them (along with other females). You don’t want to get on the bad side of these psychos or you’re finished. These types seems incurable.
If you want a glimpse of this stuff, check out the movie Welcome to the Dollhouse (girl on girl bullying).