When I was a first-year teacher, I generally called the principal Mrs. Martineau in front of the students and Joann in private, but in time I found myself calling her Mrs. Martineau more often than not.
It resurrected in me the old habit of calling adults Mr. and Mrs. My wife thinks I'm old-fashioned for it, but I really don't want our daughter calling adults by their first names. It creates a false familiarity and inappropriately levels the field between people of widely disparate social status. Calling a teacher the more formal "Miss Smith" instead of "Miss Rachel" reminds the child that the teacher is not a buddy or a peer, but someone to be treated with deference and respect.
Maybe we don't need to be as formal as other cultures, where all social interactions are last-name-only, save for the closest familial relations, but I think we've gone too far in the other direction.
I always insisted on being called by my last name when I was a teacher. I didn't discipline anyone who called me "Dave," but I reminded them they were supposed to call me "Mr. Learn." The worst reaction that ever got was from one of the more immature children, who insisted that if it was respectful for him to call me Mr. Learn, then I should call him "Mr. Cofre."
Don't remember how I resolved that one -- I might have told him that the honorific was because of my position, but I also might have started calling him Mr. Cofre until he begged me to stop.
I never called any teacher in high school by first name, except for Tom Montleone -- and I should add that he probably was the least respected teacher in the school too. In trying to be our friend, he lost claim to the respect he was owed as our teacher and instructor.
In college it seemed weird to have faculty insist on being addressed by first name. A few years a professor of mine whom I still keep in touch with periodically asked me to call him "Howard." I can't do it; he's still "Professor Marblestone" to me.