the teacher will be fine, if the goal is just lecturing. students, on the other hand, will need more that sheer willpower if they want to absorb anything lectured.
i think the sweet spot should be lesser than 40 as I had my struggles learning in a class of 40ish.
i tend to use the pain scale to give me a hint on decisions like this. so i guess the first question to ask before the pain scale is “am i in a state of shock?”
note: i am not in any medical field nor had intensive training.