We aren’t discussing a set of facts involving “in school.” If we were, this would be an entirely different conversation.
Now, your example just begs the question. Maybe “social media posts outside of school often result in school disciplinary action” have been an encroachment upon the free speech rights of the kids. Neither the school nor the government have the authority to mandate of students, while away from school, to speak to others in a manner Jesus would approve.
Kids have a free speech right to bully off school property and away from school, where the bullying is lawful. Free speech doesn’t protect only that speech that is pleasant. Free speech protects speech that offends, that hurts another’s emotions, that angers, pacifies, placates, etcetera. Yes, bullying sucks, but the arm of the public school cannot stretch beyond its four walls to punish speech and neither can it stretch beyond offsite, school sanctioned events.
It is inconsistent with the free speech clause that public schools have the authority to micro manage kids’ speech made away from and off of school property.