The United States is a country that advocates freedom of religion. The Establishment Clause and Free Exercise Clause, included in the First Amendment of the Federal Constitution, not only regulate the relationship between state and church, but also state people's freedom of religion. These two clauses have a great impact on religious activities in schools. After examining legal precedents, the author illustrates three principles of religious activities in public high schools. These principles are: (1) It is inappropriate for schools to stipulate the common form of prayer and bible reading. (2) High school students are allowed to organize bible clubs or arrange club meetings under certain circumstances. (3) Public Schools authorities should not hold an invocation or benediction during graduation ceremonies. The decision of the U.S. Supreme Court is based on the following reasons: (1) the original background and spirit for the Establishment Clause and the Free Exercise Clause and the Free Exercise Clause of the Federal Constitution, (2) the personal and sacred nature of religion, (3) the attribution of limited open forum in public schools. The decision-making of the court is moving gradually form " wall of separation" or "Jeffersonians" towards "accommodation". More and more religious activities are expected in American public high schools in the future. The U.S. Supreme Court has revealed some principles for religious activities through its legal precedents. However, whether some special religious activities are allowed on campuss still troubles school administrators.