Richard Feynman is generally regarded as a great teacher, and I'd agree, based on my experience when I had him in sophomore physics at Caltech. However, he sometimes despaired of the teaching enterprise. He said something to the effect that teaching a concept is either unsuccessful (in the case of a poor student) or unnecessary (in the case of a good student who can pick it up by reading). I'd like to suggest that there are not just two, but four kinds of students.
The poor student is unable or unwilling to learn the material.
The mediocre student will learn as much as the teacher presents, but no more.
The good student will continue to learn after leaving the teacher.
The excellent student will surpass his or her teacher.