The primary reason New Jersey’s proud high school football tradition is plunging toward irrelevancy is the growing concern about head injuries, many coaches and administrators said . That aligns with national data collected by Roger Pielke Jr., director of the Sports Governance Center at the University of Colorado, who has been studying the decline in football participation.

Football remains the nation’s most popular sport for boys ages 14 to 17. But since 2009, when a record 1.14 million athletes participated in 11-player high school football, interest has fallen. The sport drew 1.03 million participants in 2017.

Though Pielke is careful about ascribing direct causation, he notes that the biggest drops occurred after the 2012 suicide of Hall of Fame linebacker Junior Seau (who was later found to have C.T.E., the degenerative disease linked to repeated blows to the head) and the release of the movie “Concussion” in 2015.

“You always want to be careful calling something a ‘peak,’” Pielke said. “But it is robust enough now that I’m writing up all this data.”

Mike Warnock, the head football coach and a math teacher at Metuchen High School, does not need national numbers to understand what is happening at his school. Metuchen recently forfeited a game for the first time in its program’s nine-decade history. The reason? Not enough players.