So, at a modest 2 hours of instruction per day, and assuming people spend on average 12 years in education and 48 years in the workforce, you need at minimum for 1/16th of your adults to be teachers. Not 1/16th to be employed in education but just teachers and just for this education scheme. Today it's about 1/40th (in the us) and that includes post-secondary and administration, logistics etc. so you're scaling up the education sector by 3x or 4x AND you need to find something to do with the kids for the other 5-6 hours when their parents are working.
>It's worth it, since educational spending is close to the most effective public spending imaginable - something in the $3-$5 return per dollar spent
Marginal ROI does not work that way. Even assuming you get that return on what you're spending now, that doesn't mean the marginal dollar invested in education is getting that return nor that you can scale up the amount you spend and continue getting that return.