I was thinking of Nadia Boulanger, with Astor Paizzolla as the distinguished pupil. Piazzolla was trying to be a classical composer, but Boulanger said his classical music was lifeless, it was his tangos that had fire.

Perhaps the multi-talented young pupil faces perilous choices about where to focus his energies. Whether the older great teacher can warn effectively against the common errors probably depends on having a breadth of experience, perhaps having put in many years as a mediocre teacher, following up pupils and noticing how things worked out. The teacher's own youthful errors might be uniformative even if severe.