There can, of course, be no doubt that New York’s program of daily classroom invocation of God’s blessings as prescribed in the Regents’ prayer is a religious activity. […] We think that the constitutional prohibition against laws respecting an establishment of religion must at least mean that, in this country, it is no part of the business of government to compose official prayers for any group of the American people to recite as a part of a religious program carried on by government.

Hugo Black (1886-1971) American politician and jurist, US Supreme Court Justice (1937-71)

Engel v. Vitale, 370 U.S. 421, 424-425 (1962) [majority opinion]
    (Source)

This case ruled that organized school prayer was unconstitutional. The prayer in question, to be recited by each class before their teacher each day, read: "Almighty God, we acknowledge our dependence upon Thee, and we beg Thy blessings upon us, our parents, our teachers and our Country."