what do I do? Should I officially cut her off?

My cousin Anna is dating one of my exes his name is Derek and well I didn't give her permission to do that. Reason being because he is a horrible influence. He uses people for attention. And so she lied to me and said that she blocked him and stopped talking to him after we broke up. So since she lied, I cut her off. She is mad now and she is spreading lies about me. Well my close friend Annie sent me an email and told me about Anna and I showed the assistant principle about email and he said that this is not something that I should be talking about in school. I'm worried they will tell my parents about this (Me and my parents don't have a good relationship at all). The assistant principle keeps asking for more details so I am giving them. I don't understand how I am showing him what is going on but he turned it around on me.

Junior school can be a total nightmare!!!