Go to your school counselor. This girl is at best suicidal at worst she will take you and anyone else down with her.
This is one of those situations where it is ok to tell on her. Her suicidal tendencies alone are reason to speak to someone about her. She is screaming for help. I know the teen code is not to rat out another teen but this is the exact situation where you do this. For her and for you. Tell your parents, school administrators -- get her the help she needs.
Not as crazy as your situation - but my daughter had a friend that told her "don't tell anyone..." she was cutting herself. I went to just say hi to my daughter in her bedroom and I could just tell something was bothering her. She didn't want to tell on her friend as she had promised not to. My daughter finally told me when I pressed her. I let her know this is exactly when you need to break a promise -- in a situation where someone is getting hurt or could get hurt.
I immediately called her mother --- her mom got the girl help and she is doing very well now and thankful that my daughter told on her.
This situation is dangerous for you, this girl, and everyone else as school - do not keep quiet.