My brother-in-law attended Philips-Exeter, which for those not in the know, is about the ne plus ultra of American prep schools. This would have been circa 1970. His father attended Exeter in the 1930s, and his grandfather was a master there for about 30 years. I asked him if they wore "preppy" attire when he was there--lots of pink, lime green, patch madras, etc. He replied, "Good Lord, no. This was 1970. We were all in jeans. That stuff was all a creation of the Official Preppy Handbook." I will say that we did use the term "preppy" jocularly when I was at prep school in the 1950s.
I believe the movie "Love Story" did much to popularize the term. When I mentioned having seen the movie, my best friend remarked, "When it was over, did your date say, 'Let's go, Preppy?'" At the end of the movie, as poor Jenny was dying, everyone in the theater was weeping, it seemed...except me. I had to choke down my laughter at the grief over the demise of that obnoxious girl. I asked my pal what he thought of someone who laughed through the end of "Love Story." "A heartless brute," he replied. "Alternatively, a man of refined sensibility. In your case, BOTH!"
On a different note, if anyone is on the quest for the "perfect popover," Mercer offers them. Mercer shirts are very hard to beat if you like their style, I think.