>by age 18, to be able to (A) read with understanding materials commonly encountered, (B) write clearly, e.g., as well as on HN, and (C) know basic arithmetic, percentages, areas, and volumes.
Nah -- you aren't being radical enough!
For example, it isn't necessary to know the multiplication tables in order to read mathematics at university.
The argument about basic knowledge (also known as 'gateway knowledge' i.e. stuff you need to know in order to learn most other things), is back-to-front.
If there's something you're interested in, you will learn whatever gateway knowledge is necessary to learn it or learn it more fully.
e.g. football -- you will learn to count in order to keep score
e.g. multiplayer games -- you will learn more language in order to communicate with other players
e.g. harry potter -- you will learn more english in order to read the books
e.g. selling goods at market stalls -- you will learn multiplication
In each case the learner will learn more quickly and more efficiently than in the classroom. He will learn whatever it is 'just in time' and he won't be burdened by stuff he doesn't need. And he will remember it, as required, with no testing and no exams.
So, it isn't necessary to force people to learn basic stuff. We're almost certainly wrong about what constitutes the basics, anyway. (Curricula change very slowly, according to fashion, and are shielded from criticism.)