The two professions are not comparable. For a dentist, the priority is offering medical procedures with a minimal risk of infection. For a teacher, the priority is communicating well with the children in their charge in order to educate them. A dentist's mask does not interfere with the medical procedure, it makes it safer. A teacher's mask will interfere with communicating well with children, and doesn't enhance the educating procedure in any way. Finally, the dentists I personally have seen, have greeted me with an open face and friendly smile, put their masks on to fiddle about with my mouth, and then removed the masks again when I am getting off the chair in order to tell me clearly what (if anything) they have found or done, what (if anything) needs doing next, when they would like to see me again, and to bid me goodbye. They remove the masks without being asked, because facial masks are a barrier to communication!