I’m so sorry about your friend. First of all, do not feel obligated to someone just because they are having a hard time and they are hurting themselves. You can be there for them, but you don’t have to guilt into being with them because of it. That’s not fair at all. I would gently be honest with them. Explain to them that you aren’t ready to be in a relationship but offer a friendship.
If there is a school counselor to talk to about this, it may be good to have someone help step in with some guidance for you and her. That way you can have help without it escalating.
It’s understandable to be hurting and be going through things but it’s never okay to pressure or manipulate someone else to do something by using self harm or other things against them. That’s not fair to anyone.
So I definitely would seek out a school counselor to help you with this given situation. So you don’t have to handle it alone. And she can be handled with care by someone trained to handle these situations. Or even a teacher or a principal.
I’m so sorry you’re both going through this. And thanks for reaching out. We are here for you! Keep us updated okay! We want you both to be okay.