I think this is a really tricky case. The parents' Constitutional arguments are pretty compelling to me. It is one thing to say the school can control speech on campus to maintain the educational environment, another to say the school can control all speech. What if the student wants to speak out on religious or political issues? Doesn't allowing the school to police this interfere with the parents' authority to guide their children? Does anyone really believe that this particular very teenage rant off hours, off property was going to interfere with schooling or even with the softball or cheer teams? I certainly don't.
On the other hand, as someone who has spent the last 10+ years going to high school parent briefings for athletes and hearing about social media concerns, I get some restrictions. Trash talking other schools, players on other teams or even your teammates on social media is unsportmanlike conduct and something that is detrimental to the high school sports. The student in question was not suspended from school, she was suspended from the cheer squad and as the school has always told me, "participation in sports is a privilege". The coach made the call and coaches have a lot of discretion around students who present behavioral issues on the team. While I think the coach's response was overboard, telling someone who says "F cheer" to take a hike from the cheer team doesn't seem outrageous.
I want the court to say "students have rights" and at the same time say "saying f-cheer is a legitimate reason to get yourself kicked from the cheer team". Not sure how they can thread that needle.