Isn't -kun used for addressing males? Why is Takahata-sensei calling Asuna as Asuna-kun? It might probably be an error in part of the scanlator group..
No, teachers use -kun to address all students, regardless of gender. They generally won't use the -chan ending, which is way to familiar especially for a male teacher to a female student (it would be, frankly, creepy). They probably also use -san but I don't really pay attention to stuff like that so I don't recall.