"You're not my friend"

We have been battling the Kindergarten "mean girls"! Lately, Caroline has been saying "You're not my friend" when she gets angry with me. She also says, "I don't love you today, but I'll love you tomorrow"! I knew this was coming from school as this is not something I have ever taught her to say. I've tried to explain to her that everyone is her friend and sometimes people make you angry, but it is usually just a mistake and that they are still your friend. Well come to find out she has been saying this to just about all the kids in her class as well as at home. I finally got so sick of it, I told her she would get a time out if she said it again. They other day, she almost said it, but caught herself at the last second and instead said "I don't like... your sweater"! She also had to apologize to everyone in her class that she has said this to. Her teachers say this is so common, especially with the girls in the second semester of school. I knew this was something that she would encounter probably in the 3rd and 4th grade, but dear Lord, it now starts in Kindergarten?! She is so excited about her birthday party this weekend, but with what's been going on at school, I'll be surprised if any of her friends show up!