one entrant per academy, so Hal missed the Slams
Hal had missed out on the soft grass, clay, and Har-Tru surfaces of the Jr. Slams, because a singular disadvantage of attending a North American academy is that O.N.A.N.T.A. rules for Jr. Slams permit just one entrant per academy in each age-division, and John Wayne got the nods.
Context
The governance apparatus deciding which bodies get which surfaces. Hal "missed out on the soft grass, clay, and Har-Tru surfaces of the Jr. Slams" because "O.N.A.N.T.A. rules... permit just one entrant per academy" and Wayne got the slot. The rule shapes the very development of the body — a player's feel for varied surfaces is rationed by an entrant-quota, so Hal's game is quietly deprived of the grass and clay Wayne got to learn on. The institution allocates athletic experience itself.