***NOT A TEACHER***
I would say:
I've never seen a student
tofall asleep during class.
Here it's not clear which class it is. It can be a biology class or a class on Shakespeare. It doesn't really matter because you're speaking in general terms. "During a class" full stop doesn't sound quite right to me (unless it's clear from the context which class it is you're referring to). What does sound right to me is:
I've never seen a student fall asleep during a class on Shakespeare.
Let's see what others think of this.
Student or Learner