Some of the slogans for teachers are, "We cannot teach people anything. We can only help them discover it within themselves" by Galileo Galilei. "Prosperity is a great teacher; adversity is a greater. Possession pampers the mind, privation trains and strengthens it" by William Hazlit.
There's the old standby, "Teachers do it with class," but in light of all those scandals in the news, that might now have a negative connotation. How about, "Teachers - they're definitely not in it for the money."