It's a strange assumption that schools exist to provide cannon fodder for professional sport. I always thought that they existed for the purpose of education and that sport and other activities such as music and drama were to develop the mind and body in other aspects of life, promote values, comradship etc.
Also a strange notion that because something is old it is out of date. Wimbledon's been going for a while and part of its strength is its tradition.
If schools in each of the association's really wanted change, it would happen. For example, if any GPS school thought that it was too old or outdated, they would simply leave and join ISA. The idea that some "big brother" should come in and tell people who they should play and what organisation they should be in is a bit strange. Freedom of association still exists in Australia.
Another thing that some rugby people need to understand that there is more to sport than rugby. Each of these associations play a variety of games against each other and rugby is only part of the picture. The whole of the GPS, CAS or ISA do not revolve around rugby.