More kids than ever are attending state-funded preschool in the U.S., 1.8 million of them the last school year. A report by the National Institute of Early Education Research finds that 37% of 4-year-olds and about 10% of 3-year-olds are enrolled. However access to free preschool remains uneven across states. California contributed significantly to the increase by making all 4-year-olds eligible for a program called “transitional kindergarten.” But the rapid rollout meant the state missed many quality benchmarks last school year. Evidence shows that high-quality preschool can positively impact a child's future, but challenges such as uneven access and funding persist.

Teacher Angela Hall always starts the day gathering her preschool students in a circle in their Shreveport, Louisiana, classroom. The kids gig…