I often tell people that as much as I wish I could find a time machine and meet Mozart face-to-face, I would not want to be his student. I have read that Mozart was a very good and loyal friend, but as a teacher, he was very critical.
Here, on October 23, 1777 (age 21), he writes to his father about Nannette Stein, the daughter of his friend Herr Stein, who played for him.
Whoever can see and hear her play without laughing must be a stone (Stein) like her father. She sits opposite the treble instead of in the middle of the instrument, so that there may be greater opportunities for swaying about and making grimaces. Then she rolls up her eyes and smirks. If a passage occurs twice it is played slower the second time; if three times, still slower. When a passage comes, up goes the arm, and if there is to be an emphasis it must come from the arm, heavily and clumsily, not from the fingers. But the best of all is that when there comes a passage (which ought to flow like oil) in which there necessarily occurs a change of fingers, there is no need of taking care; when the time comes you stop, lift the hand and nonchalantly begin again. This helps one the better to catch a false note, and the effect is frequently curious.
Oh, did I forget to mention that the girl he criticizes is only eight years old?!