Preschool Drama

Tonight, as N was getting ready for her bath, I heard her shriek, "You're not my friend and I'm going to kick you in the face."  Based on her tone, she was mad as hell at her father and she said it to be mean.  I ran into the hallway, scooped her up, and put her in the time out corner so everyone could calm down.  I was so mad at this display of anger from her.  Where did she learn such horrible words and when did she learn how to be mean?

After she calmed herself, I sat down with N and asked who used those words. She instantly shared the name of one of her classmates and based on what I've seen from this little girl at after school, I believe her.  She then proceeded to tell me how the same girl won't play with her anymore and said they weren't friends.  Apparently, N responded with a "well, you're not my friend either" comment. For reference, N is 3 and the other girl is 4.  Seriously?!? When did preschool become filled with drama?

N and I had a long discussion about using nice words and not mean words.  She told me all about how some girls won't play with her friends and it's clear that there's some exclusion happening.  We talked about the importance of playing with everyone and making good choices.  She seems to understand, but I'm sure in the thick of things at school, it will be hard to remember.  Any guidance on how I should proceed?  I'm at a loss here...