The claim that public schools are “religion-free” zones is a Religious Right myth that has no basis in reality.
Public schools can (and do) teach about religion. Teachers discuss its role in world and U.S. history. They talk about biblical allusions found in great works of literature. They lecture on how religion has influenced art and music.
The approach must be objective and tied to legitimate educational objectives. Proselytism or elevating one faith over others has no place in the classroom.