Here is an education mystery that gets far
too little attention: if proponents are correct
that attending preschool dramatically improves
the academic and social development of
all children when they enter kindergarten and
grade school, why don’t we see higher graduation
rates and test scores later on? Proponents
claim that a dollar invested in early education
returns anywhere from $2 to almost $14 to
society. If that’s true, why haven’t we already
seen evidence of this return on the massive
increase in preschool investment that has
occurred over the past 20 years?


The study wiliki cites is a high intensity family intervention program and not just preschool for needy kids.  We have invested heavily in preschool education in the past 20+ years with no discernible difference in graduation rates and test scores.

https://object.cato.org/sites/cato.org/files/pubs/pdf/pa641.pdf