from ed.gov:

Although the Constitution forbids public school officials from directing or favoring prayer, students do not “shed their constitutional rights to freedom of speech or expression at the schoolhouse gate,” [ 9 ] and the Supreme Court has made clear that “private religious speech, far from being a First Amendment orphan, is as fully protected under the Free Speech Clause as secular private expression.” [ 10 ] Moreover, not all religious speech that takes place in the public schools or at school-sponsored events is governmental speech. [ 11 ] For example, “nothing in the Constitution … prohibits any public school student from voluntarily praying at any time before, during, or after the school day,” [ 12 ] and students may pray with fellow students during the school day on the same terms and conditions that they may engage in other conversation or speech. Likewise, local school authorities possess substantial discretion to impose rules of order and pedagogical restrictions on student activities, [ 13 ] but they may not structure or administer such rules to discriminate against student prayer or religious speech.