What is this hypothetical 12-year-old going to do for the next six years until they are old enough to get the kind of minimum-wage job that accepts people without a diploma? Child-labor laws are still a thing...

We're running into the other problem with public schooling: it's not so much about education as warehousing children while their parents work during the day, until like a tree, they grow the requisite number of rings that magically grants them status as an (more-or-less - see alcohol, car rentals, etc.) independent being. You can see this with the expansion of organized after-school sports and activities - school usually runs from 7:30-8:00AM to 2:30-3:00PM, except parents are still working until 5:00PM, so you've either got to trust your kids to get off the school bus and not burn the house down in two hours, send them to grandma, pay for ruinously expensive child-care, or hope little Johnny or Sally is into soccer/football/baseball/band/art, etc.

On the first point, I would argue that understanding how money works is a pretty fundamental mathematical competency - percentages like that come up all the time with sales taxes, meal taxes, etc, and they are usually not built into the prices on the sticker/menu.