In a Korean Hanja class (Chinese character) class, teacher was in late 20's, early 30's. I was a student (70+ years at the time. Fellow student (grad student, late 20's) introduces me as his "friend, who is an attorney." Teacher was a Ph.D. candidate.
Throughout the semester , we have recitation doing problems at the end of the chapter. So we go around the room, each student doing a problem. Teacher called on each student by first name followed by ssi. But he always said daeum -- "next" when it was my turn. He knew what my name was because I wrote it on the homework and quizzes. I finally figured out in the middle of the semester that in the Korean hierarchical system, he didn't know how to address me. By age and profession, I should be spoken to honorifically always. But in the classroom, he was my superior because I was a student. So to resolve the ambiguity he just called me "next'".
On the other hand, in two other classes, we had visiting full professors from Korea. They both told me to address them by their first names -- no need for 선생님 of 교수님.