Kings College, a school Gelling described as "the rugby union diehard school", competed in championship grade at this year's New Zealand Rugby League Nationals.
Former Warrior and Sport Nation host Anthony Gelling said the participation reflects schools responding to competition for young players.
He recalled that some schools once told pupils, "if you want to go play league, you're not playing for us." Now, he said, union schools are offering league programmes because students want to play both codes.
Gelling also warned that rugby league's domestic profile has weakened, saying people may not know who won the Stormont Shield or Fox Memorial.
He supports the NZRL's goal of building a domestic pathway so 15- and 16-year-olds need not travel to Australia for competitions such as Harold Matt, SG Ball or Jersey Flegg, but said establishing it will take time.
He said league was once widespread, then contracted after the pre-war and post-war periods, remaining strongest in Auckland and Canterbury while union spread nationwide.
Gelling expects both codes' systems to change, adding, "I'm excited to see where it goes because it could go to a place where none of us are anticipating."