How do you handle an obsessive ex?

I have an ex who keeps contacting me despite my requests for her to stop bothering me. We broke up in early November on very bad terms, and I made it clear that I could not have her in my life. Since then, she has pestered me through FaceBook, Instant Messenger, email, and phone. I have ignored all of her calls and messages and blocked her on IM and FaceBook, but she contacted me through her sister's FaceBook. I replied and asked her sister to ask her not to contact me, but my ex still keeps contacting me. I'm still a high school student (as is she), and I'm very scared that she'll show up to my house or my high school. I honestly don't know what she's capable of, but I fear that she'll try to hurt me or my friends. She threatened to shoot me and everyone who had ever been interested in me, and I've seen her at some of my local hangouts that she'd never been to before we met. I've been contemplating a restraining order, but I don't know if this sort of behavior qualifies. Does anyone have any advice for how to handle this sort of situation?