This is a country music styled joke. A lot of country songs are built around a repeating chorus that in the last verse changes its meaning. This joke works the same way. It’s the Conway Twitty of comic strips.
FEMALE: Yes, I prefer good students.
CZAR: And you think female students are better than male?
FEMLE: I've seen evidence.
CZAR: Isn't that a little biased? It certainly doesn't continue outside the classroom.
FEMALE: I see evidence.