I doubt it's the charisma. My class is... well, we number 32, but I have never seen all students in attendance except in his classes. Those who are in attendance seldom pay attention, often going so far as just talking at a normal volume during a lecture, except again in this particular teacher's class.
It's not even the subject, I don't think. People always say how amazing as a teacher he is, and he was valued among the highest according to this inquiry from two months ago.
Must be high CON. It seems almost impossible he's hardly been ill in all that time.