Preschool can provide a boost, but the gains can fade surprisingly fast (washingtonpost.com)

That's known to be true of Head Start.[1]

[1] http://www.edweek.org/ew/articles/2010/01/20/18report-b1.h29...

Just another example of the iron law of g immutability.

Grade level schools, adopted in the USA in the middle of the 19th century, essentially dumb down those who are a bit more advanced at an early age. It takes a few grades to destroy the good work of parents preparing their preschoolers, but the playing field has pretty much been leveled by 3rd grade. :(

https://en.wikipedia.org/wiki/Prussian_education_system#USA

I would also question if most of the primary education is useful. Some of it is useful, but not much of it for the number of years put into it.

Plus students who are good at an early age are dumbed down by this kind of education system.

Personally, only secondary education made any significant impact on my career.