I feel somewhat uncomfortable with characters that have their eyes closed most of the time, because I feel as if they're impossible to see through and that they're cunning.....
That type exists - like Xellos, or Hayami in Minami-ke - but a lot of those characters are surprisingly benign. Like Yachiyo in Working or the Kazuho in Biyori. So it's more like ... they are either cunning or very harmless.
I'd say it's clear where the teacher belong. Hm. Well. Kind of. I suppose she's dangerous in one specific way ;)