She then announces that she's bi, and she has a crush on a girl at school....
I am not in any way saying that some girls arn't bi or that somearn't really suicidal...you have to consider what it's like to be a teenager today. Victorian letters and literature are filled with love notes to each other.
While its unlikely that most of these were sexual relationships in Victorian society - in our sex centric society it is not unbelievable that simply the barriers have been dropped (and the knowledge is there).
All of us are married (to men), most of us with kids. Teenage men are the lowest form of life on the planet... You must therefore treat and protect her as if she is bisexual (because she is. At least.) Respect her girlfriend, set the same sexual boundaries you would with any potential partner (and realize that they will ignore them at every opportunity, just like any teenage partners) and be there to pick up the pieces and buy the ice cream and give her lots of love after the inevetable breakup.
Dan Savage had an advice peice to a college student who was going through angst over whether to tell her parents she was bi. And be prepared to do it all over again in 6 months after another love interest who may be male or female. Of course i have no isse with homosexuality or bisexuality (as it appears niether do you), my concern if i were in your shoes however would be that she is falling into the rising trend/fad of "bisexuality" in middle schools everywhere.
You seem like a wonderfull parent, I just know that if it were me in your daughter's predicament i wouldn't tell my mother.... Or maybe, just maybe, more people are coming out or experimenting because it's less likely to get them beaten than it used to be.