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I don't think it's all that uncommon at most schools. (At least it isn't at my school) There are still people I call by their last names that I'm not even friends with anymore.

I would think Brain and Angela are fairly common names and if you really want to make sure you get your point across or their attention you use their last name.

I agree with the point that common first names lead to people being referenced by their last name. Last names are more specific. I experienced that phenomenon myself. Plus, to a teenager, using last names probably seems like an adult thing to do.

The references to Angela spending a lot of time at Sharon's and Brian's houses could mean a couple of different things. The first thing that comes to mind is that there is a continuity lapse in the writing. The second thing that comes to mind is that there is no continuity error; teenagers tend to exaggerate things. A minute seems like a day, an hour a week, etc. The third thing that comes to mind is the Angela's different worlds theory, which someone mentioned earlier. Angela spent time with Sharon and Brian mostly separately. The three of them are in closer proximity in high school.

I would think Sharon and Brain we closer when they were younger and had a mini falling out. (Obviously not as cataclysmic as Angela's)
Not to say Sharon is doing it on purpose but she has a social life and can't be bothered with Brain. (And Brain probably has more of a sister relationship with her anyway.)

I was always called by my last name in high school. Of course, it might have had something to do with the fact that I have a twin brother and noone could tell us apart. . .

Funny thing is, people in college called me by my last name, too - even if they had never met my brother or knew I had one.

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I think people who have charismatic or easily pronounced last names are also the people who tend to be called by their last names. And I agree with the common thing, if the last name of a person was something really common like Li, for example, then it would be unlikely you'd go around calling them that. Also, I think it's a thing Jocks do a lot, because during their sports, announcers refers to them by last name. I think in the scene where Sharon and Kyle are outside the gym before Kyle's practice and she is bugging him about why he asked her out, a teammate calls him by his last name. Last names can be used affectionately, because someone who knows your last name generally has to know you on a pretty familiar basis. Or it can be used when you want to distance yourself emotionally when you are talking to the person (as we see Rayanne do to Sharon (Cherski)...basically it seems to take many forms. But hey, it's as much your name as your first, so why not? No one ever called me by my last name because it's too hard to say right, and it doesn't have that ring to it.