Gas Mask guy is probably also a witch, considering how we know witches can tell other witches apart, hence why the ones we know so far (Teacher, Girl, etc) are able to identify them so quickly, and how the Inquisitor in question has their hand always covered, and seems to be missing and arm.
It's also suspect that he happens to appoint himself to the room where we know the most witches are in. Sure, it could be that the teacher is gone, but they'd just get a substitute who's more qualified. In truth, he's probably aware that there's witches in the classroom
you know, at least they should act a bit less surprise because it may review their identity and be admirable towards the crisom like normal people would