And here's the text in another book, courtesy of Google Books, on page 264:

https://books.google.com/books?id=TuvOng_Yh6wC&newbks=0&prin...

The story could still be made up, but the source for the quote is this book:

Art & Fear: Observations On the Perils (and Rewards) of Artmaking

by David Bayes and Ted Orland

https://www.amazon.com/Art-Fear-Observations-Rewards-Artmaki...

Here's two that prove it.

[0] - https://blog.codinghorror.com/quantity-always-trumps-quality...

[1] - https://www.amazon.com/dp/0961454733/?tag=codihorr-20

prove what? That someone printed the anecdote in a book?

What would suffice, video recordings of every class?

Naming the teacher, the school, the year, the city, something that would give a hint for someone to follow up for verification.

And here's audio of the authors saying that it's a true story:

http://www.innonavi.com/wp-content/uploads/2017/04/David-Bay...

That's a repetition of the same claim, not a source. He doesn't mention simple information like who or where the class was, which there is no reason to be secret about.

Also, it doesn't pass the obvious sniff test, that a teacher would spend a whole semester giving half of his class a terrible experience that uttery failed to teach them anything, leaving them with "a pile of dead clay" as the author claims.

To be fair, there exist sites like snopes.com which do this type of fact-checking. But I couldn't find it there.