The most extensive and careful study of preschool to date shows a slightly negative effect in the longer term:
By the end of kindergarten, the control children had caught up to the TN?VPK [preschool] children and there were no longer significant differences between them on any achievement measures. The same result was obtained at the end of first grade using both composite achievement measures.
In second grade, however, the groups began to diverge with the TN?VPK children scoring lower than the control children on most of the measures. The differences were significant on both achievement composite measures and on the math subtests.
This more or less supports Arnold Kling’s null hypothesis that educational interventions make no difference.