From Wikipedia:
“Track and Field News (T&FN) has tracked records by any American high school students, in any competition until they enter college. These records include marks made in open competition against higher-level competitors, post-season meets and international competition up until August 31 following their high-school graduation.”
Unfortunately this doesn’t address when a student technically transitions from one grade to the next. August 31 seems too late for that since many schools are back in session by that date.