Last I knew, Zatopek and Beamon were not high schoolers at the time.
If the OP means "greatest performance by U.S. high schooler," it unquestionably would have to be Bob Mathias's win in the Olympic decathlon: a 17-year-old establishing himself as the world's greatest athlete. NO CONTEST.
Michael Carters National High School record in the Shot Put, 81'3 1/2" (24.78 meters) Carter set this mark at the 1979 Golden West Invitational (Sacramento, ...
I was glad to see someone posted Michael Carter because that is the only correct answer… To put this into perspective for the people of letsrun who know nothing about field events, in 1979 he launched the 12 pound shot put 81 feet 3.5 inches. That is right at 4 feet .75 inches further than the 2nd best throw in high school history (Ryan Crouser). His throw is 4.997% better than anyone else in high school history…. Hobbs Kessler’s 1500 meter is only .818% better than Alan Webbs converted 1500… compare any other record with 2nd best and you won’t come anywhere close to 4.997%. Numbers don’t lie my friends. The record will never be broken.