I'd recommend a meeting with the principal and faculty. Someone should speak on behalf of the teachers. Are you a member of the teacher's union?

The principal should then publicly rescind the no-tuck-ban and say, "Now that I have your attention, students, let's talk about what is appropriate for school attire...." This way, he will save face.

Stand your ground. If you need to, involve the school board. Outside of requiring school uniforms (I would jump to wear khakis and polo shirts everyday with sneakers if I were a teacher!), clothing in good taste should be acceptable for school attire, for both students and teachers.