Girl drama, kindda need help on this one?

So I've been friends with this girl for almost 3 years, and all through out junior high we were in separable. But she can be a very fake person. I used to tell her everything. This one time I was going through a very hard time and after school I went to my locker and looked over and there was this group of people watching me and my friend was in that group, but I thought nothing of it. Then I opened my locker and this very inappropriate drawing was in it and everyone including her was laughing and I ran home crying. And this other time she told someone something very personal about me and next thing I knew the whole school knew and they were teasing me for months about it. And this other time I had sex with someone and I only told her and my other really good friend, I told them to keep it a secret. Then what do I know, she told 3 people with the biggest mouths ever and still continues to tell people. She's been a backstabbing liar and has got people to hate me, but then whenever she sees me she acts all innocent. I might be switching schools, but that's such a big change. Please help me, I need advice...