If a school district is facing low enough enrollment that it can no longer maintain two schools, one public and one separate, the FIRST school to be shut down should be the separate school, not the public.  The public is the general use option, that all faiths can use equally, and should be the last school standing in a district, no matter what.

So what happens when a district gets attendance figures so low that there's not even enough students to justify one school, and now the board is considering merging districts and having one central school for multiple districts, which is what happened in this area?

Seems to me that centralization is the RIGHT answer.  The WRONG answer is for the parents to be selfish and work with the separate board to build a new school in their district that won't be any more viable, long term, than the one that just got closed. They save a few years of shorter bus rides only to see the school close in half a decade, anyway.