It's a really good post, and worth reading the whole thing. In section V, Scott gives an extended response to the general category of claim that school provides some kind of illegible benefit that doesn't show up in test scores.

[...]I get nervous when I see a giant institution with lots of really legible costs, whose legible benefits don’t withstand scrutiny, and people proposing a bunch of very diverse, kind of flaky sounding illegible benefits that mean we should still force everyone to participate in it.

...

There’s this weird trap a lot of adults fall into where anything a kid does on their own, however interesting, is “wasting away”, and anything they do at school, however ridiculous, is Exciting Prosocial Learning Fun Glowing Childhood Memories. I think this might be entirely a function of whether the parents can spectate and take pictures that look good on a mantlepiece: easy with hobbyhorsing, harder with learning C++.

The section strays a bit from the narrower topic of school absences, but I'm really glad it does, since he articulates the status quo bias really well.