OK, that helps a lot.
I teach in a Catholic HS, and I can tell you some of the things that "sell" our school at open house-- comments I've heard from parents (in no particular order.)
1. They're impressed by the feeling in the school- the general religious atmosphere. Our kids pray before every class, stop dead in their tracks at noon to say the Angelus, and say the 3 o'clock prayer together. None of that is a deciding factor in and of itself. But it does add up to a school where our faith is important.
2. They're impressed by our kids. Talk to the kids you pass in the halls as you tour the school. Watch how they interact with each other. Our kids (admittedly much older than the kids you'll be seeing) are very vocal about why they love their school. They're proud to wear their school jackets (a bit less so about the uniform) But they can see the school community in action.
3. Academics. This one is easy. For us, it's reflected in the fact that 100% of our kids go on to college. For an elementary school, you would want to look at the local Catholic schools they're getting admitted into. See how they do on the standardized tests.
4. Teacher qualifications. Depending on where you are, not all Catholic school teachers are required to be certifed. (In NY, there are so few teaching jobs that schools can easily require teachers to be certified.) Certification doesn't automatically mean a great teacher, but it does set the bar at at least a minimal level.
5. Class size. (OK, we don't score points here- our classes are large.) Frequently, small class size means more attention to individual students.
6. Extra curricular. Kids love school a whole lot more when there's something do do after that 3 pm bell.
7. Extra help. Our teachers are required to be available after school for least 20 minutes every day. How does the school handle it?
8. Extra fundraisers. You have no idea what a pain they are until your son or daughter brings home the third or fourth one. We have one per year: the yearbook ad drive.
9. School safety. Should be a no-brainer but sometimes isn't. Are the doors locked during the day to keep intruders out? Are visitors required to sign in or wear a pass or something?
10. Your gut. The most important. Take notes and list impressions.