What crime are we to surmise that Prairie/OA has been accused or convicted of?

It wasn't clear to me whether it was a crime or something more akin to being a non-violent but mentally ill person who might try to leave, wander off, or even get lost unintentionally. However, it's feasible she's going to be charged with child endangerment or something similar, and they think she's a flight risk. I think they've kept it vague intentionally.

As user1118321 said, I don't think the ankle bracelet was due to being charged with a crime - rather it was a (somewhat extreme) measure on the part of her parents to prevent her from leaving the house without their permission, as she has proven herself to be both mentally unstable and a flight risk.

It's certainly possible that there were kidnapping concerns - although this is not explicitly mentioned. Based on how OA's father lets her go at the end, though, I doubt criminal charges were pending. If they were, he's be opening himself up to aiding & abetting charges by letting her go, and I'd think he would at least mention that before letting her leave.

The reporters were gathered because they caught wind of the whole thing and wanted to report on it. OA was already a media personality due to her earlier disappearance and return (not to mention the whole "cured of blindness" thing) so any news concerning her would draw a crowd pretty quickly.

BBA was fired simply because of the circumstances in which they were discovered. She (a teacher) was found in an abandoned house with a woman recently recovered from a kidnapping and a bunch of younger students, in the middle of the night, with no explanation. [I say "no explanation" because I'm assuming BBA was smart enough not to tell the truth. Admitting that she (and the students under her charge) were not only facilitating but, in fact, participating in OA's delusions would only have made things much, much worse.]

The principal had already observed BBA getting unusually close to these kids at school, so finding her with them like this would've been the last straw. Rules about this sort of thing are especially strict for jobs like "teacher" because no one wants their kids to be supervised by someone who is in any way shady.