St. Anselm's usual graduating class is in the upper 20s: if the number used is 30, that gives us 256 AP tests. These are generally taken by juniors and seniors, so there are probably about 60 to 65 boys taking them. Washington International School has about 900 students from kindergarten through 12th grade. Figure that as being a bit heavier in the lower grades, and reckon between 60 and 70 students in a graduating class: a graduating class of 67 works out to 481. In both cases, the Post is giving us four digits of index for three digits of tests.
I imagine that the index can be useful as an indicator of aspirations. The school that has most of its students taking a couple of AP exams is likely to be asking more of them than the school that has almost no students taking such exams. After a certain level, though, I don't know how much it matters. I can name half a dozen or more high schools in the District of Columbia where it would be difficult to graduate without getting an excellent preparation for college. I know the challenge index only for the two above, and will likely forget it within the week.
In Peopleware, the software consultant Tom DeMarco summarizes a discussion he had with Tom Gilb, the author of Software Metrics. Gilb is or was a believer in measuring everything, DeMarco something of a skeptic. Neither man convinced the other, but DeMarco took away from the discussion what he calls "Gilb's Law":
Anything you need to quantify can be measured in some way that is superior to not measuring it at all.