I've only ever been involved with small country Catholic schools, so I've never had to do any of this.
Maybe go & talk to your parish preist & ask him? Or ask the school? They might prefer your children to be involved with the church outside school hours, but unless they have huge numbers, I don't think this would be a reason to turn you down.
I really don't have experience with bigger schools with waiting lists & high enrollment numbers though, so I'm probably not much help. Sorry.