This Supreme Court decision says students have First Amendment rights, and as long as their social media posts don't disrupt their school much or indicate cheating, bullying, and such, they have the same rights as an adult. Now of course, this will probably only apply to public schools (since the 1st Amendment just says what governments can't do). Private schools will be free to violate their student's rights all they want.
The Supreme Court actually overturned an appeals court ruling that said schools had no right to do anything about off-campus student speech. Instead they said the same rules apply for students as any adult: They have Freedom of Speech, but no shouting "FIRE" in a crowded theater. If their expression actually does harm, then they are responsible for doing that harm. So schools need to show harm, like the "standing rule" in federal lawsuits, if they want to punish student speech. No: "It pissed off the coach, so you are suspended."