I used to work for a Catholic family, whose daughter attends a Catholic school, and they needed the priest to confirm their Catholic status and attendance at Church and all that, and also they needed a 'sponser', a family who attended the Church whose child was already at the school, to ... I'm not 100% sure what they did, but they needed that 'reference' I guess. I also remember my boss stood in line for 4 hours at the school to get the application papers and fee in... and that wasn't even to get in the school, just to apply!

I think some schools are really popular, so they'll take Catholic students first, and if you aren't Catholic you just hope that there aren't as many Catholic students as there are places at the school.