Nobody is expected to do their best work every day in any conditions with any inputs. You're expected have a "good enough" outcome the overwhelming majority of

Nobody is expected to do their best work every day in any conditions with any inputs. You're expected have a "good enough" outcome the overwhelming majority of the time which is also the goal when it comes to teaching. With a good teacher all but maybe the bottom few percent should at least be learning enough to be regarded as passing and most should be passing comfortably. A not so good teacher will go through the same basic motions using the same inputs but without that hard to quantify expert touch that comes from skill compounded with experience they will not produce the same result and they may not understand why because you don't know what you don't know. This generalizes to other skilled professions. Don't get me wrong, I don't expect every college professor to be a great teacher. But the ones that aren't and don't want to be shouldn't be complaining that teaching is a chore and that some students are harder than others. That chore is what keeps the lights on.