Tricky case. I don't think the school would be getting involved if it was a private conversation. But what's the threshold for comments being "public" nowadays? They clearly heard about it. If I share a post with multiple friends, is that public? What if 90% of the school is on my friends list? The latter isn't an outlandish figure given how much emphasis some teens place on pumping their subscriber / friend numbers.
And it's not like the school is suspending her education. This is just participation in an extracurricular activity. If I mouthed off online and dropped a bunch of F bombs regarding my employer, they'd have every right to fire me. That much is stipulated in the employee handbook, in fact. How is this different? It's a private group of volunteers with, I presume, no guarantee of even making the squad. You can be asked to leave.
Honestly I'm sympathetic - everyone has bad days, and teenagers aren't known for their great judgement. Kinda wish the school had shown some leniency. They could have avoided the whole mess by just giving her a slap on the wrist - suspended for a single game, or something with a little bite. It seems ridiculous that such a trivial matter ends up at the supreme court.