To what degree should society pick up the cost of that choice?

To the degree the community allows it. If society as a whole decided on making things better, tons of things would change (including a mixed zoning and multi-family housing overhaul), but as it stands local cities and counties have vast power in managing themselves, and they have a vested interest in continuing to divert resources to their biggest income sources (single-family homeowners).

Here schools are not part of cities or counties.

When I imagine what a 25 mile dangerous bike ride looks like here, it's living 25 miles out in a place where the US highway is the only route to town. But there's a high school out there, so it wouldn't be 25 miles.

Obviously other situations will exist, but living 20 miles from the school you attend is a corner case and will often be driven by choices the parents are making.

It can be argued that single-family homes and the associated low-density land uses are only cash cows in the short term, and are in fact ticking timebombs in the long term.

https://www.strongtowns.org/the-growth-ponzi-scheme/

Wow. I would consider it unacceptable living more than 25 minutes walking from school.