"Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances." -U.S. Constitution, Amendment I
What is relevant to our purposes here is the Establishment Clause. The Supreme Court has held that for a public school to provide for the recitation of prayer violates this clause by way of the government endorsing religion through the public school system. Engel v. Vitale, 370 U.S. 421 (1962).