how can i get a psychopath to leave me alone?

at the beginning of school, i noticed there was a girl who always sat by herself and never talked to anybody. i felt like i could go talk to her and see if she wanted to be friends. at first she was cool, a little weird but i could handle it. after a while she started telling me about how she felt like killing herself and other people and acting really violent and impulsive. she would slam herself into walls and she tried to bite me several times. after a while i started to try and distance myself from her out of fear. every now and then she would see me in the hall and follow me for the rest of the day until i could shake her off by saying i had something really important to do and she really couldn't go with me. one time i was walking around with her and she lead to the elevator, i followed her until i realized it probably wasn't a good idea and tried to get out before the door could shut. she grabbed the back of my bag and yanked me back in and i screamed. i was pretty sure i was about to die. luckily, my bf came around the corner yelling at her and pulling me away from her. my bf HATES her since she always follows us and when we ask for some alone time she screams about how everyone hates her and how she is just going to kill herself. ive tried telling her to see a doctor or therapist about her anger and she yelled at me about how shes not crazy and that im an awful friend. this all sounds mellow but she is really violent and attached to me. she tried to beat up my boyfriend several times. i dont know what to do but i cant be mean to her or just ghost her. i dont want to be responsible for her killing herself if she does. she keeps asking for my address and sometimes i see her walking up and down the streets in my neighborhood. im truly afraid for my safety.