…early education does increase reading and mathematics skills at school entry, but it also boosts children’s classroom behavioral problems and reduces their self-control. Further, for most children the positive effects of pre-kindergarten on skills largely dissipate by the spring of first grade, although the negative behavioral effects continue. In the study, the authors take account of many factors affecting a child, including family background and neighborhood characteristics. These factors include race/ethnicity, age, health status at birth, height, weight, and gender, family income related to need, language spoken in the home, and so on.
In other words, you learn both how to read and how to raise hell, but a head start is useful only for the latter talent. Here is the link to the summary and study.