Tinker v. Des Moines - Students do not “shed their constitutional rights to freedom of speech or expression at the schoolhouse gate.”
More specifically:
In our system, state-operated schools may not be enclaves of totalitarianism. School officials do not possess absolute authority over their students. Students… are possessed of fundamental rights which the State must respect, just as they themselves must respect their obligations to the State. In our system, students may not be regarded as closed-circuit recipients of only that which the State chooses to communicate. They may not be confined to the expression of those sentiments that are officially approved. In the absence of a specific showing of constitutionally valid reasons to regulate their speech, students are entitled to freedom of expression of their views.