Here are a list of precedents for the case.
Lynch v. Donnelly (1984): A government may not coerce anyone to support or participate in religion or its exercise, or otherwise act in a way which establishes a religion or religious faith or tends to do so.
Engel v. Vitale (1962): It is no part of the business of government to compose offical prayer for any group of the American people to recite as a part of a religious program carried onby government.
Marsh v. Chambers (1983): The Court condoned a prayer exercise for a session of a state legislature.
Lemon v. Kurtzman (1971): A government practice must: 1)reflect a clearly secular purpose, 2)have a primary effect that neither advances nor inhibits religion, and 3)avoid excessive government entanglement with religion.
Board of Education of Westside v. Mergens (1990): The Court recognizes that, at graduation time and throughout the course of the educational process, there will be instances when religious values, religious practices, and religious persons will have some interaction with the public schools and their students.
Everson v. Board of Education of Ewing (1947): The Establishment Clause forbids not only state practices that aid one religion or prefer one religion over another but also those that aid all religions.