At my former school, I frequently referred to my principal (out of the ear shot of students) by her first name. In front of her and other teachers I'd use Mrs. I also referenced her by her first name on occasion in front of parents especially parents I felt comfortable with. This put me in an weird situation once. A parent emailed her about a topic after I had discussed that topic with the parent. The parent started the email off to the principal with the principal's first name. The principal was flabbergasted because she felt like she didn't know this parent well. I know the parent only did it because I had referenced the p as her first name numerous times with this parent. I don't think the principal ever realized that it was me who was at fault because I simply shrugged it off as "oh well, you know how she is.". I do think that the use of last names has its place and should be used when appropriate.