A class can be an exploration of new territory, the cleaning of a dusty attic, a pair of thick new glasses, or a gentle reminder of some things you already knew. It can be a challenge, a maze, possibly even an affront. Unless the instructor is inept, it is a joint venture where both parties learn and each emerges enriched.
I've taught a lot of students over the years and they have taught me. I'm not sure what it is but whenever a subject is well-explored, there is always something else at work in the room.
A spirit, perhaps, but a nice one.