Apparently, this year, we reached gender parity in schooling on an international level. In other words, if you select a random woman and a random man across the world, the expected value of the number of years of schooling they receive is the same.
We can still see problems in the distribution. As Alex Tabarrok notes, there are a small number of countries where women receive significantly less schooling offset by a large number of countries where women receive somewhat more schooling. I would argue that certainly the countries where women are massively undereducated have a major problem with educational equality, and it's likely that the countries where women are educated somewhat more have structural problems with educational quality (not missing the e) - the US is an example, where it's been well-documented that the structure of the American classroom is very poorly suited to male brain development.
I don't know what the "optimum" level is*, but you'd expect to see it within a reasonable margin of individual country-by-country parity, and that is clearly not the case in the data. Still, worldwide parity is not a small achievement given where the world was just a century ago.
*ideally, you'd want everyone to have exactly as much education as they want, which you'd expect would line up approximately with the productivity value of education, with a fudge factor for the fact that school is more fun than work but less fun, for most of the world, than doing neither work nor additional school. There are four reasons I can think of why this may not line up into perfect parity between genders on education:
1. We don't know biological determinants of intrinsic desire to be in school for more time, and assuming the substantially wide distributions among both men and women are centered at the same point is no more valid an assumption than assuming they're different in either direction. You would expect them to be close to each other, though.
2. A woman exits the work force to have children for much longer than a man does so may on some level value education less because it results in less net additional productivity over a lifetime.
3. Disease rates, war/accident fatalities, and other such catastrophes may make the length of the female working life greater than or less than the length of the male working life, which would result in different average education levels because of different net additional lifetime productivity as a result of additional education.
4. Chosen labor types differ between genders. Men and women differ physically (duh), and neurologically, men and women perceive numbers and words differently, and also perceive risk differently, which may lead to different career choices even under perfect freedom. Education may differentially affect productivity of those careers. For example, if communication skills can be learned outside of school but math needs to be learned in school - and this is a hypothetical, i don't know if it's true - you would see more people intending math-heavy careers be in school for longer. At the most extreme case, hard manual labor, for which men are better equipped, may see differential (probably lower) productivity benefits from schooling.
hat tip for the link to Alex Tabarrok at Marginal Revolution.