OS I think you are wide of the mark with the presumption "that the best CAS lads tend to be destined for the NRL before they even join their CAS school team" I'm interested in who has played CAS and now is in the NRL besides Connor Watson who attended Knox (Katoa is obviously destined for it). As far as I am aware far more players have left GPS schools for the NRL than CAS. Even then it is a small number comparatively over the years. Scots - Crichton, B Smith, Newington - Moeroa, Milne, Lussick, Murray, Kula, Weekes, Kings - Sua'ali'i, Penisini, Daniel Conn. Riverview - Darcy Lussick, Joeys - T Wright, C O'Kane.
Rudderless also noted back on page 67 I think that 10 of the NSW CIS Rugby League 18s team are from GPS. 2 are from CAS