In West Virginia Board of Education vs. Barnette, decided in 1943, the Supreme Court decided that no one could be compelled to say the Pledge of Allegiance.
In Engle vs. Vitale, decided in 1962, the court decided that teachers and school administrators, as agents of the state, could not lead students in prayer.
In Lee vs. Weisman, decided in 1991, the court expanded its doctrine in Engle and declared that outside clergy could not be brought in to say prayers at official school events. In doing so, the court created the doctrine of coercion, which stated that the rights of nonreligious students were violated if religious activity took place in a public realm, even if they were not compelled to participate.
Finally, in Santa Fe Independence School District vs. Doe, the court expanded the coercion doctrine declaring that students could not lead prayers at official school events.
If the Supreme Court had decided that saying the Pledge of Allegiance was a religious event, then the doctrine of coercion would require them to strike out the words "under God."
Although they dismissed the case in 2003 on procedural grounds, the issue will likely surface again.