(Your link to Public education as share of GDP seems to be recursive to this post.)

Really good questions. I also wish there was better historical data, including for the many centuries of history where the one-room-schoolhouse/tutorial method dominated. Very hard to say how many people attended school in Ancient Greece, the proportion of those people relative to various demographics, or even just how big schools were.

Quick and very incomplete tidbits off the top of my head, from a combination of pitch decks and my history of education class:

• something like 70% of children > 5 and < 18 in the US were in public primary schools as early as 1880
  • the delta between that and present numbers is largely about the expansion of high school attendance
• pre-20th c. US public education spending was something on the order of a thousandth of a percent of today's spending (after adjusting for today's dollars)
• today there are about 100k public schools and 30k private schools in the US, and those numbers have been steady for a couple of decades
• today roughly 1 in every 1000 adults in the US is a professional teacher in primary or secondary education