The State of Global Learning Poverty: 2022 Update [pdf]
unicef.org
unicef.org
As far as I could get it is "attends school" and they flirted with it in "Reading, together with writing, numeracy, and socioemotional skills, is a building block for all the other education outcomes that societies care about."
Socioemotional learning stood out in there, not because of the SEL criticisms, but because it seems incredibly Western to think that there is a Correct Way that applies to the entire world.
So it's a low-threshold indicator of basic literacy.
[edit]: Here's the wording from p 5:
"One of the most intuitive indicators of the learning crisis is the learning poverty rate, which measures the share of children who cannot read a simple text with comprehension by age 10. "
I don't know about past investments. But I found one part of the report that shows the potential return:
Page 11:
>Without action, the current generation of students now risks losing $21 trillion in lifetime earnings in present value, or the equivalent of 16 percent of today’s global GDP. Relative to current incomes, this economic cost is disproportionately borne by low- and middle-income countries, in which this generation of students could lose $11 trillion of lifetime earnings.9 In addition to this intergenerational inequality shock, evidence is mounting that the shock has worsened inequality within the current generation of children, as those from lower socioeconomic backgrounds and other disadvantaged groups have suffered larger learning losses.10 Putting this as annual earnings equivalent, in low- and middle- income countries, this implies that the average person of the school-age generation might see a reduction in annual income of $975.