Why are kids mean??

I had a horrible grade school flash back just a couple of hours ago. My daughter is so upset, 2 of hers friends told her they didn't want to be friends with her , then this evening to top it off one called her and told her she was a b**ch, she is heart broken. I am heart broken....
Why are kids so mean?? I have always tried to tell both the kids you don't have to like everyone but be kind, is being kind so hard?? I wasn't always kind when I was a kid either, I think that is why I try to make sure they are kind. Now I am not an idiot I know they aren't perfect and when away from me I have no idea how they act but at least we talk about things like this and I try to use them as learning tools.
SO she doesn't want to go to school tomorrow and I did the mom thing and said well you are going and you just act like it doesn't bother you and pray to tonight before you go to bed for God to give you strength, I fully believe He will, she knows it, and has a good relatonship with Him. (between me and you, I wanted to say okay you can stay home, remembering my horrible days)