I interviewed some students about the incident and, though I heard nothing about mental problems, the students did mention that the boy and girl have not liked each other for quite some time. The fight itself began as follows:
"This is an American map," I said as I was trying to illustrate regional dialects.
"American MAP," the boy said, with a strange pronunciation of "map."
"American MAP," the girl said, imitating him.
"Don't copy me...!" responded the boy.
I don't know any of the words that came next, but they were certainly not kind.
I should explain my feelings a little more clearly, as my original post (below) seems to dwell on my anger. It is true that I felt angry with the students, especially since they were disrespecting my classroom. But my deeper feeling, the one I didn't share with them, was sadness. When harmony is breached there seems to be no limit to what people will do here.
Last spring I saw a man being chased down the street by an angry mob. He tripped and fell, the mob surrounded him and kicked him mercilessly. It took 2 or 3 minutes before police arrived to break up the crowd and carry the man away.
Why did today's incident bother me? Why am I upset about this behavior? Probably because my students are like my younger brothers and sisters. I'm 27 and they're 19 and 20. What does it feel like when you see your siblings fighting? Terrible. Especially when there's no reason for it.