Missing the Mark
There are many sayings that are befuddling when you’re young. You piece together wording from the conversations you eavesdrop on, and sometimes they don’t make sense until you get older and learn more about the world (and proper grammar usage).
This person wasn’t the only one to make this mistake. It’s an easily relatable error if you think about it from their perspective; they were likely aware that Catholicism was a religion, and they saw that it was in front of the name of their school because Catholic institutions are typically named as such. So why wouldn’t you assume that a public school follows the public religion?