I have been cleaning out school boxes of things I have saved over the years as I prepare for my retiring in June. I pulled out this old, always inappropriately used quote from one of the manila envelopes: "He who can, does; he who cannot, teaches." I don't think there is a teacher alive or dead who has not had that demeaning, dismissive statement hurled at them directly or at the profession generally at last once in their careers.
The statement is from George Bernard Shaw, but it is only the first line of what he actually said. Of course, those who use the line to insult educators are either ignorant or mendacious, as the full quote puts the statement in a much more teacher friendly context.
Here is the full quote, with the context in italics. Let the reader be the judge:
"He who can, does; he who cannot, teaches. What is more they are the only available teachers, because those who can are mostly quite incapable of teaching, even if they had the time for it."