I find the following recurring law school scenario both humorous and full of nervous tension:

STUDENT raises hand.
PROFESSOR: Yes, what is your name?
STUDENT: Adam.
(Awkward pause.)
PROFESSOR: Hm. Adam. (Gingerly.) Actually I prefer to use last names.
STUDENT: Oh. Um, my last name is Phillips.
PROFESSOR: (Smiling.) Ah. Mr. Phillips!
STUDENT proceeds to make uninsightful comment.

First names are generally more informal than last names. That is, if you call someone "Adam," you're probably on a more familiar basis with him than if you call him "Mr. Phillips." Our culture dictates that there are certain situations where it is appropriate only to call someone by their last name, until invited to use their first name. That's the origin of the phrase, "Please! Call me Kimberly!" (Or whatever your first name happens to be.) Generally, once you have been invited to call someone by their first name, you do so.

Law school is, in my experience, the only exception to this general social trend.

In my example, Adam Phillips has invited the professor to call him by his first name. There are very good reasons why he may have done so. Maybe he just doesn't like his last name, or else particularly likes his first name. Maybe, as a younger, less experienced person than the professor, he feels that it's more appropriate for him to go by his first name while addressing the professor by his last name (as, for example, children do with most of their elders--especially teachers). Most likely, he just feels more comfortable using his first name, because that's what almost everyone calls him, and he doesn't even think about it--his name is "Adam," not "Mr. Phillips."

But the professor doesn't want to play that game. The professor rejects the student's invitation to informality and insists on calling the student by a name the student doesn't want to be called, feels uncomfortable being called, and isn't accustomed to being called. It creates even more awkwardness when the next student is asked her name. Her knee-jerk reaction is "Kimberly," but she catches herself (visibly) and says "Smith." Or even more awkwardly, "Ms. Smith" (after considering whether she should call herself "Miss" or "Ms."). The teacher at this point is forcing people to call themselves by a name nobody ever calls themselves. "Mr./Ms. Lastname." It's terribly uncomfortable.

Why do law professors do this? Are they trying to make the students uncomfortable? Are they attempting to show respect for us by ignoring our explicit desires and treating us like grownups anyway? Does using first names make them uncomfortable?

To their credit, there are some law professors who purposely call students by their first name. Or better, who ask you to tell them "what name you'd like me to call you." If I ever teach, that's how I intend to act.

I had one professor my 1L year who awkwardly called everyone by their last name. After the class was over, I started to develop a relationship with her, and now I'm working for her as a research assistant. Back in class, she used to call me "Mr. Astle." But now, it's never anything but "Matt" (though I still call her "Professor [Lastname]"). Has our relationship changed? Well, yes, a little. We know each other better. But we're still in a very distinct student-teacher relationship. Why is it "Mr. Astle" in class, but "Matt" outside of the classroom?