The current education systems are hurting kids.<p>I don't know how preschool looks like, but when I suspect to have a less constrained (less or no tests and grades) learning environment then I think, is this shaping the expectations of learning in general and that the kids simply get bored of school and its constraints. As the study results say, the "negative effects" are seen in second and third class, which is the time things get tight in school. Also likely that the use of advanced teaching methods in preschool also leads to higher expectations of school.
Which of course has no universal applicability. There are pre-K programs that work, especially where the parents are taught critical skills to help their children develop. See, for example, Harlem Children's Zone.