““Culture” means … the cultivation of the mind, the taking care and improving of the native faculties of the mind in accordance with the nature of the mind. Just as the soil needs cultivators of the soil, the mind needs teachers. But teachers are not as easy to come by as farmers. The teachers themselves are pupils and must be pupils. But there cannot be an infinite regress: ultimately there must be teachers who are not in turn pupils. Those teachers who are not in turn pupils are the great minds.”

Le temps est un grand maître, dit-on; le malheur est qu'il soit un maître inhumain qui tue ses élèves.
Letter written in November 1856, published in Pierre Citron (ed.) Correspondance générale (Paris: Flammarion, 1989) vol. 5, p. 390; Paul Davies About Time: Einstein’s Unfinished Revolution (New York: Simon & Schuster, 1996) p. 214.

Part VI, ch. 29 (Samuel Bester)
Up the Down Staircase (1965)

"Getting to Know You", The King and I (1956).

As quoted in The Eclectic Magazine Vol. VII, (January - June 1868)
Variants:
The teacher who is attempting to teach without inspiring the pupil with a desire to learn is hammering on cold iron.
As quoted in School Arts (1935) by Art Study and Teaching Periodicals, p. 91
A teacher who is attempting to teach without inspiring the pupil with a desire to learn is hammering on a cold iron.
As quoted in Making Minds Less Well Educated Than Our Own (2004) by Roger C. Schank, p. 151

From a letter to Edna Mann (October 30, 1926)
Letters

Source: Thus Spoke Zarathustra

LXXX. TEACHER
Orphic Sayings
Context: The true teacher defends his pupils against his own personal influence. He inspires self-trust. He guides their eyes from himself to the spirit that quickens him. He will have no disciples. A noble artist, he has visions of excellence and revelations of beauty, which he has neither impersonated in character, nor embodied in words. His life and teachings are but studies for yet nobler ideals.

Source: Aphorisms and Reflections (1901), p. 31