To give a specific example: I recall being taught "how" to play a given passage, working on it diligently all week, only to find at the next lesson that it was not right. What was not right? Most likely, everything was not right, including the teacher's ear. For that, too, changes.
It has to change, otherwise the teacher would be dead.
Every moment of every student's life is teachable just as every moment of every one's playing is potentially entirely musical. Denying the vitality of the moment deprives all concerned of empathy and ultimately of growth.