Perry Educ. Ass'n v. Perry Local Educators' Ass'n
In Perry Educ. Ass'n v. Perry Local Educators' Ass'n, 460 U.S. 37, 45-46 (1983), the U.S. Supreme Court held that when government maintains a traditional public forum or creates a limited public forum for debate and expressive activity, any content-based censorship must survive strict judicial scrutiny. Strict scrutiny is the most stringent standard for free speech, and few restrictions on speech can survive strict scrutiny.
When a public facility such as a school is not a public forum, by tradition or by design, "the State may reserve the forum for its intended purposes, communicative or otherwise, as long as the regulation on speech is reasonable and not an effort to suppress expression merely because public officials oppose the speaker's view." Perry, 460 U.S. at 46.