The Single Sex Colleges (with boarders) have a lot more boys playing football than SMC.
In primary school for example, we have 108 boys in year 6 & 7 where as Sacred heart has over 300.
In the past, Year 8 at SMC jumped to somewhere between 250 and 300 boys. Now we are co-institutional (Girls only Year 8 and Boys only year

, the number of boys in grades 8-12 will decrease possibly as much as 50% over time.
Rostrevor, Saint Peters, PAC, Sacred Heart are boys only colleges with boarders and hence they have greater depth. They also have a stronger base of footballers whereas SMC's heritage of scholars is evenly based between soccer and footy.
Even Primary Schools are changing. St Peters college is triple streaming from next year. This will increase their strenght in primary sport for boys.
You are also correct that Independent Schools and Public Sector Schools play a knock-out competition. I remember watching SMC defeat Sacred Heart at Norwood Oval when Cornes represented the Marists and Tredrea/Ebert played for the Maroons (SMC).
This year, I think (stand to be corrected) that Sacred Heart played in the GF but I am not sure if it was against Henley High or Rostrevor. Someone out there will no doubt know. remind Tom we have an excursion tomorrow
