Just a reminder, the mods have asked us to keep this on topic, i.e. reply with direct advice to the OP's situation. This is not a thread on the rights and wrongs of suicides.
To the OP, I express my sympathies. I can't really imagine what it's like and I hope I'll never experience it. Your post has given me some pause to reflect on whether I too, am one of the negative pressures on my students. As everyone else has said, treat her as normally as possible and don't talk about it openly with anyone.
That said, if you have the chance to spend more time one on one with her, I suggest just showing her a little extra teacherly love. Whenever there's a chance encourage her feelings of self-worth or share some hope, just in a small and subtle way.
Also, you mentioned some drama with an ex-best-friend. Before writing her off as a demon, think about the scrutiny and judgement she must feel towards herself, or that may be coming from those around her. It would be very easy to feel the blame in this situation, and she may even end up being blamed by other students. With that sort of negative self-view, she may just sink lower and lower into bad behavior. I suggest that you treat her as you used to and even show her some extra-friendliness once in a while. If she's never given a chance, how can she feel sorry and reform?
This is the age where children start to form their self-identities. If they get the message "I'm a bad kid" they'll adopt it and live up to it. If they get the message "I'm worthless" that's how they'll think about themselves. I still remember how much just one good or bad word from another person could affect me at that age--especially if it came from an adult I respected. So for both of these students, I think the best thing you can do is be supportive and positive as much as you can in your interactions with them.