My personal feeling is that if first names are used, as in other professions eg nursing and medicine, familiarity breeds contempt.
I agree with that in most professional situations. I once had a surgeon who, when he rang, or wrote to, me, would either say 'This is John' or sign his letters 'John'. I don't want to be on first name terms with someone that I might need to tell off at some point! It makes it harder once that line has been crossed. I prefer professional distance (if that makes sense) including in the classroom.
I taught adults and although I would always introduce myself to a new class as Mrs XX XXXX, they'd still call me Miss lol (and some of the women were older than I was).