I'm pretty sure the felony prosecution is bogus, considering what Polk county is like in general. But I'm going to set the race debate aside, since no one is going to change any minds on that subject anyway. What I'm looking at are:
1) The explosion was "small" but was apparently big enough to be heard pretty far away. This wasn't a test-tube "pop" experiment with hydrogen. On the other hand, no damage to people or property was reported. So "was it bigger than a firecracker?", "was it bigger than a cherry bomb?". And what prosecutions for other commercially available "explosive devices" have taken place in Florida in the past? You know those cases must be out there. Fireworks are sold legally in most parts of FL and all it takes is an adult stupid enough to provide the fireworks to an equally stupid kid.
2) She elected to do it on school grounds and not in the science lab or at home. I think this was more of "let's reproduce that YouTube video" and less "science experiment" than is being claimed. OTOH, it clearly was an _experiment_ and not an attempt to actually damage anything.
3) We have no information on what, if any, safety precautions she had in place. Being a teenager, probably none, but we don't know.
4) There's a strong hint that she's protecting another kid here. If she was just doing it on her own, she could have done it nearer to her home. I bet they'd drop all charges if she coughed up a name, and it wouldn't surprise me if the overblown response is just a form of coercion because the authorities have the same suspicions.
From my POV for a expulsion, I'd be looking for something like a 1/4 stick of dynamite sized explosion, and for a felony prosecution, I'd be looking for actual damage to body or property. There's certainly enough intent and stupidity on her part here to justify a suspension and maybe a misdemeanor prosecution for attempting to cover it up as a "science" experiment, but I don't see evidence to go beyond that.