Jake Vigdor is Associate Professor of Public Policy Studies and Economics of the Terry Sanford Institute of Public Policy of Duke University. Their latest study looks to see if K-6 is better than K-5 for children. The Duke research has a chart that shows a significant improvement in behavior in the K-6 environment vs. the K-5 and that that behavior change carries forward for several years.  It also mentions that the students were behind in scores and caught up during the sixth grade in K-6, although that finding needs more thought.  There is also the factor that if you configure schools K-6, 7-9, 10-12 the ninth graders would have a more immature social structure and possibly reduce some of the issues connected with being a ninth grader in a high school.