https://www.fcd-us.org/the-effects-of-oklahomas-pre-k-program-on-school-readiness/
The report shows that pre-K improves academic achievement among all children regardless of income, race, or ethnicity.
For students as a whole, the overall effects of the Tulsa pre-K program are: a 52 percent gain in the Letter-Word Identification test score; a 27 percent gain in the spelling test score; and a 21 percent gain in the applied problems test score. That is the average change in each test score attributable to the Tulsa pre-K program, above and beyond the gains that naturally occur as the child ages one year.