Or even your colleagues in general? At the school I am leaving, I always used first names, which is pretty typical of most of the people there, except I might use a last name if it is in front of a class or something. It seems like at my new school, everyone says Mrs. and Mr., for higher ups, secretaries, and maybe other teachers (haven't met too many yet). It seems weird and artificial to me - I don't like it. Maybe it will change as I am there longer. What about at your schools?