Earlier quoted context omitted.
I don't see an issue with it legally/constitutionally. Case law has been pretty clear that it's not an unrestricted right, this punishment seems logical and thus likely acceptable under common law, and I haven't seen any real support for a theory in which this is not acceptable (besides 1A MFer).
While students do not retain all their First Amendment rights, case law is clear that they retain them to some extent, and in particular Tinker makes clear to what extent schools may restrict those rights. Further, I believe the school will not be able to successfully argue that her speech posed a substantial disruption. As it was neither bullying nor threatening and shared privately, the extent to which it may have…
I doubt the privacy laws really hold snapchat as private communication. PA law is very restrictive about recording. If someone recorded her conversation so that it could be played beyond the 24hr period in snapchat, it's possible they could be prosecuted for a felony under the wiretap laws if snaps are actually considered private.
Tinker doesn't really provide clear criteria. The disruption would have to be proven, but the articles don't give us all the details around the issues that were being caused on the team. It's possible she reiterated or confirmed her position at school or the activity, further causing the disruption even if some other person was the first to bring it up (ie it's likely not all of her speech occured off campus). Tinker also doesn't define censoring. It's mostly about not suspending or suppressing student speech on campus in a formal classroom setting. The nature of an extracurricular can carry additional responsibilities and expectations (things like illegal activities, bullying, not following team standards, even performance issues - which you could say this meets) . In this case the school did not try to censor the off campus speech, it merely took action to remove a team member that had a performance issue by violating team policy. There was no formal discipline and she was even allowed to continue with softball.