Just as a null result in Swedes[1] and Danes[2] demonstrates there is no effect, at least at the level of highschool.
But even if we step away from the beaten question of pre-uni education ROI and study the real and measured Flynn effect, we can sadly conclude that it is mostly (at least after the 1950s) about so-called[3] hollow gains. The problem with these being that they do not generalize to most tests and low-level measures of g such as reaction time. It can be argued that "Flynn IQ" can be of at least some use[3], but even then, the effect is close to saturation[4] by now, and increases to the underlying g-factor would avoid the controversy altogether.
Historically, real & massive g-loaded IQ gains tended to occur when people for the first time ceased being nutrition- and sanitation- limited during their formative years. If we sidestep the question of IQ heritability, there are lesser-known potential ways[5] of increasing g in children. Clearly, this field could use much more money and human resources allocated to it, given the unquestionably valuable prosocial outcome it could deliver.
I'm all for methods to raise g, especially in healthy adults and not just in children to be born sometime in the future, but just pushing more education into anxious highschoolers (or mid-career adults, for that matter) isn't going to cut it. Some fundamental approach based on neuroscience of learning and memory[6] is sorely needed.
> Just as a null result in Swedes[1] and Danes[2] demonstrates there is no effect, at least at the level of highschool.
Why would that demonstrate there is no effect? No treatment -> no effect bolsters the case for education, as my example shows treatment -> effect. There was no change in education that would have led to large IQ increases during that period in those countries. There was in South Korea, and the effect on IQ was dramatic.
