I agree with you, i think he should demand that his students not yawn in such a disrespectful way and should be asked to leave if they are simply too tired to manage being there.

Though i also disagree a bit.. his class room, his rules. He should demand a dress code for his class if he doesnt' care for how they dress. But... he probably won't see skin if he doesnt' allow them to continue to yawn and stretch.