My kids are in a catholic school and they are Anglican. You dont have to be catholic to go, but they do have to be christened and they take a copy of there baptism certificate. Its a popular school, and it just keeps the numbers down.
Thats our story too. We too have a popular school.
I think it depends on numbers and vacancies. Our school has 3 classes per grade of 25+ kids.

Goodluck with your decision.

Jacqui