Here are a list of precedents for the case.
Lee v. Weisman (1992): In disallowing an invited religious speaker from delivering a prayer at a public high school graduation, the Court said that the Constitution forbids the State to exact religious conformity from a student as the price of attending his or her own high school graduation.
Lemon v. Kurtzman (1971): A governmental program 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.
Lynch v. Donnelly (1984): The Court observed that merely a reference to God does not violate the Establishment Clause because it is part of our religious heritage rather than an endorsement of religion.