Teachers have the right while in school to express themselves to fellow employees as long as such communication takes place in the faculty room or otherwise out of hearing of students. The Establishment Clause is not violated when school personnel communicate between themselves about religion; it can only be violated when they communicate improperly with students about religion. Teachers who share religious convictions may always have private conversations about religion with other employees or pray together so as long as students are not included and cannot overhear their conversation or prayer.
Teachers must use this free expression right wisely, being careful not to express themselves to other employees who do not welcome such religious interaction. As in all workplaces, once a fellow-employee tells a teacher that his religious communication is unwelcome, his continued religious communication with at employee could be considered harassment. If a teacher’s religious substantially disruptive to the work environment, he could be prohibited from such religious expression. All school staff, including bus drivers, have the same rights and restrictions as teachers with regard to students and other personnel.
—Keeping Christ in America’s Public Schools, Gibbs & Gibbs, 2008