From the majority opinion:
In our system, state-operated schools may not be enclaves of totalitarianism. School officials do not possess absolute authority over their students. Students in school as well as out of school are "persons" under our Constitution. They 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 Keyishian v. Board of Regents, 385 U.S. 589 MR. JUSTICE BRENNAN, speaking for the Court, said:
"'The vigilant protection of constitutional freedoms is nowhere more vital than in the community of American schools.' The classroom is peculiarly the 'marketplace of ideas.' The Nation's future depends upon leaders trained through wide exposure to that robust exchange of ideas which discovers truth 'out of a multitude of tongues, [rather] than through any kind of authoritative selection.'"
The principle of these cases is not confined to the supervised and ordained discussion which takes place in the classroom. The principal use to which the schools are dedicated is to accommodate students during prescribed hours for the purpose of certain types of activities. Among those activities is personal intercommunication among the students. This is not only an inevitable part of the process of attending school; it is also an important part of the educational process. A student's rights, therefore, do not embrace merely the classroom hours. When he is in the cafeteria, or on the playing field, or on the campus during the authorized hours, he may express his opinions, even on controversial subjects like the conflict in Vietnam, if he does so without "materially and substantially interfer[ing] with the requirements of appropriate discipline in the operation of the school" and without colliding with the rights of others.
Under our Constitution, free speech is not a right that is given only to be so circumscribed that it exists in principle but not in fact. Freedom of expression would not truly exist if the right could be exercised only in an area that a benevolent government has provided as a safe haven for crackpots. The Constitution says that Congress (and the States) may not abridge the right to free speech. This provision means what it says. We properly read it to permit reasonable regulation of speech-connected activities in carefully restricted circumstances. But we do not confine the permissible exercise of First Amendment rights to a telephone booth or the four corners of a pamphlet, or to supervised and ordained discussion in a school classroom.
It can hardly be argued that either students or teachers shed their constitutional rights to freedom of speech or expression at the schoolhouse gate. …The problem posed by the present case does not relate to regulation of the length of skirts or the type of clothing, to hair style, or deportment. It does not concern aggressive, disruptive action or even group demonstrations. Our problem involves direct, primary First Amendment rights akin to “pure speech.”