Oh, I have no doubt she was just trying something out, but it's hardly guaranteed to be harmless. Worst case, you might mix something like bleach and ammonia, and kill half the school.
Can't tell without further details, but the fact that she was surprised is scary. I wouldn't expel her, though, just give her a long and serious talking-to.
Wow, alarmist much? Two seconds of googling suggests that it makes chloramine, which isn't exactly mustard gas ( http://chemistry.about.com/od/toxicchemicals/a/Mixing-Bleach... ).
As for not knowing what the mixture will do--well, there's a really obvious way to find out, right? We're all about TDD here.
No way would that kill half the school. Might hospitalize some people in the room, but that's the worst realistic scenario.
The school officials might know it, but I'm not so sure about the cops. It seems she was expelled due to the school system's zero tolerance policy. What about the felony charges? That was a separate decision by the cops or the District Attorney. I doubt violating a school policy requires the filing of criminal charges.
Most school administrators, in my experience at least, seem to be zealots and jump at the chance to put students "in their place".
I doubt it was a decision based on race. She's obviously a "good" kid, and the cops and admin know it.
What's bullshit is that she was doing a very low-risk "experiment". It doesn't really sound like a real experiment, just a curious kid trying something out. Or at worst, the kind of harmless prank everyone used to do. But it was harmless, and she was probably just doing it for fun, or because she was curious.