On the one hand, the free exercise clause (“Congress shall make no law … prohibiting the free exercise” of religion) seems to support Mattson’s view that public school teachers should be free to express their religious identities at work. On the other hand, the establishment clause (“Congress shall make no law respecting the establishment of religion”) seems to argue against any sort of religious dress for public school teachers, on the grounds that impressionable students might read such garb as an indication that their school is pro-Catholic (if their teacher wears a habit), pro-Sikh (if their teacher wears a turban), or pro-Muslim (if their teacher wears a hijab).
I believe that the Constitution requires public schools to allow teachers in upper grades (where students are generally less impressionable and less deferential to authority, and typically cycle through various teachers each day) to express their religion in dress, as long as they maintain in their teaching the religious neutrality required by the Constitution. However, I believe that the Constitution requires public schools to deny this same freedom to teachers in lower grades because here, students are generally more impressionable and more deferential to authority, and often have only one teacher in a given school day.
My position might seem convoluted, but it is no more so than many U.S. Supreme Court rulings on religion, which are forever trying to balance the demands of the establishment clause for religious neutrality with the demands of the free exercise clause for religious liberty. My position is also in keeping with the views of many ordinary Americans, who continue to differentiate themselves from Europeans when it comes to religious tolerance.