Up until university, really, kids don't really have a huge say in where they're going to school, and the choice of a religious education is up to the parents. As for why parents might choose a religious school for their kids, it could be physically closer to home or work; public schools in the area could range from abysmal to subpar; or any other sort of non-religious reason. I've not known the local religious schools to be exclusive to members of their faith, either, so ultimately the proportion of religiously interested and theologically educated students coming out of these schools is really low, I'd guess 10-20%.