This is a friendship problem and I want some help?

There is a girl and she used to be my friend but I have never liked her personality because she is very bossy and selfish. Recently, a new girl came into our friendship group and she thinks that the new girl is replacing her. She talked to the teacher and said that there are already problems between us. But after talking to the teacher, nothing changed. One evening, she rang me and she was crying so much while my othere friend said that she is so depressed that she wanted to suicide. And the new girl said it's all her fault and she will move school. I didn't know what to do and I thought I should be nice to that girl and maybe I misunderstood her. But today, she did it again. A while ago, she borrowed my USB,and there are pictures of me and my family but I didn't want to show her and unbelievablily, she stole the picture and send it to my other friend. That friend told the teacher but somehow she got away with it. And today, she asked me for my phone and I was reading a novel that include some sexual things in it and when u saw she was really into it and she even ask my whats the novels name. I thought she is reading for the whole time but when she bend down, I realized she is in my message, and when she noticed that I saw what she is on she said she accidentally go in. And I said ok then she go on with reading. After a while, she wanted to write something so she went into the drawing pad software. There was a picture I drew before and I think its embarrassing to show anyone so I wanted to grab my phone back but then she started pulling on the other side and telling me to show her. This continued for a few minutes and she gave in. I was so surprised by what she did. It's my phone and j can't even do what I want ? There are a lot more things that she has done and so me and the other friend decided to write a letter to a teacher and see what we can do. But now Im not sure if what I did was right.