Answer: The establishment clause
Explanation: The Establishment Clause is a limitation placed upon the United States Congress preventing it from passing legislation forcing an establishment of religion, broadly making it illegal for the government to promote religion with taxpayer resources.
In 1962 the U.S. Supreme Court ruled on the application of the establishment clause to prayer in public schools. In a 6-1 decision known as Engel v. Vitale(brought by parents of ten pupils in a school district where prayers were said), the Supreme Court ruled that the prayer was unconstitutional as a violation of the establishment clause of the First Amendment.
It doesn't matter whether the prayer is nondenominational and voluntary, or that it was voted on and led by students, it still involved indirect coercion as it was implemented by the public school district and therefore the establishment clause applies to the school district above too.