I've been wondering about this for a long time, but I haven't been able to find a clear answer. According to Finn Laursen, Executive Director of Christian Educators Association International, "U.S. courts have held that school leaders may not require prayers or religious content from speakers, nor invite religious personnel to lead such activities. However, while maintaining religious neutrality, under the First Amendment school officials cannot rescind students' freedom of speech."
Does this mean that it would not be legal for a school to invite a Christian minister to say a prayer at graduation? The state university where I work has done this in every graduation ceremony I have attended. If I am reading the above quote correctly, even this Christian advocacy group wouldn't consider this legal.