Except when their rules violate the Constitution.
The issue of school speech as it relates to the First Amendment of the U.S. Constitution is one that has been of much debate and the subject of much litigation since the mid-20th century. The First Amendment's guarantee of freedom of speech applies to students in the public schools: In the landmark decision Tinker v. Des Moines Independent Community School District, the Supreme Court formally recognized that students do not "shed their constitutional rights to freedom of speech or expression at the schoolhouse gate."[1]
The core principles of Tinker remain unaltered, but are tempered by several important decisions -- Bethel School District v. Fraser, Hazelwood School District v. Kuhlmeier, and Morse v. Frederick.[2] Despite respect for the legitimate educational interests of school officials, the Supreme Court has not abandoned Tinker; it continues to recognize the basis precept of Tinker that viewpoint-specific speech restrictions are an egregious violation of the First Amendment.[2] In Rosenberger v. Rector and Visitors of the University of Virginia, the Supreme Court declared: "Discrimination against speech because of its message is presumed to be unconstitutional." Rosenberger held that denial of funds to a student organization on the sole basis that the funds were used to publish a religiously oriented student newspaper was an unconstitutional violation of the right of free speech guaranteed by the First Amendment. Accordingly, for other speech, that is, on-campus speech which is neither obscene, vulgar, lewd, indecent or plainly offensive under Fraser nor school-sponsored under Hazelwood nor advocating illegal drugs at a school-sponsored event under Frederick, Tinker applies limiting the authority of schools to regulate the speech, whether on or off-campus, unless it would materially and substantially disrupt classwork and discipline in the school.