Plus one for "how do you know?". My only caution to add in the case of teachers is that since the problem (or benefit) with standards is that there are so many of them, and since one corollary is that when a standard is chosen it's hard to change later and experimentation is rare, then the situation is doubly worse for teachers whose jobs revolve around a standard of performance because unless they have tenure the risk of a failed experiment aimed at helping the students learn (as opposed to only getting a high score with some performance standard) is too high. Anyway, your comment immediately reminded me of this anecdote I had saved in my quotes file that I thought was apt:
'One day when I was a junior medical student, a very important Boston surgeon visited the school and delivered a great treatise on a large number of patients who had undergone successful operations for vascular reconstruction.
At the end of the lecture, a young student at the back of the room timidly asked, “Do you have any controls?” Well, the great surgeon drew himself up to his full height, hit the desk, and said, “Do you mean did I not operate on half the patients?” The hall grew very quiet then. The voice at the back of the room very hesitantly replied, “Yes, that’s what I had in mind.” Then the visitor’s fist really came down as he thundered, “Of course not. That would have doomed half of them to their death.”
God, it was quiet then, and one could scarcely hear the small voice ask, “Which half?”'