I teach at the school I attended. I started working there 5 years after I graduated, so most of my former teachers were still there. I absolutely could not bring myself to ever call any of them by their first names. Ever. Now I work with mostly people who are my age or younger, so I don't have trouble calling them by their first names. My former principal was a teacher in the building when I was a student. I still remember her calling me one summer to ask a question, and she said, "Hi, this is FirstName." I still can't use her first name, and I've worked with her for 20 years.