No, they are not. Tinker and Rosenberger show that schools have the right to censure conduct or language (both considered speech for purposes of the constitution) if that language is profane or vulgar on campus and which tends to disrupt school discipline whether on or off.
And yes, people can be sued for stating opinions that conflict with others' opinions, depending on the language used. It happens all the time. Again, our free speech rights are governed by time, place and manner restrictions. One can be sued for defamation, for inciting a riot, for intentional infliction of emotional distress, like the cases where people are sending messages to the parents of missing kids, stating their loved one is in hell, crying for them, etc.
In this case, the time, manner and place restriction would apply to conduct of student while acting as a student. The kid tweeted that she told the governor to his face that he blows and sucks, at a school-sponsored event. Whether true or not, she can be disciplined by the school for such conduct, due precisely to the language she used, but not for her opinion. That is the difference.