It seems that the topic of how to define "old school" gaming has arisen once again, much like the mighty Cthulhu rising from ancient R'lyeh to plague mankind with misfortune. I'm certainly surprised as to how much time and effort is wasted upon this debate when the answer to what is old school was settled once and for all back in 1979.
It probably slipped under most people's radars, but that year saw the publication of the Dragon Dudes Handguide for Advanced Dragons & Deep Pits (TPR Games). Within that tome, on p. 113, the subject of old school gaming and how to determine if your current campaign and/or ruleset qualifies as worthy of the old school label was addressed. Frankly, I'm shocked that James Maliszewski has overlooked this, as he's displayed much scholarly knowledge about the history of this game of ours. Perhaps the unknown, hideous, black oil of Dwimmermount hath mazed his mind. The fact that Jeff Rients hasn't beaten me to mentioning this book is also quite perplexing, since Jeff is usually the go-to guy for rpg miscellanea and weirdness. One too many folding chairs to the head in a steel cage match, Jeff?
Luckily, this humble author is here to take up the slack. I spent the morning going through my old books that had been packed away in storage. Due to my extensive training as an archivist, it only took me four and a half hours to find my copy of Dragon Dudes Handguide and to scan the relevant page. Seeing how TPR Games went out of business some years ago, I feel comfortable posting this scan here without the fear of legal ramifications.
Now that the method of determining whether or not your game is old school with 100% accuracy has been established, please go out and enjoy your summer. I know that I certainly intend to enjoy mine.