This study reports the effectiveness of a developmentally based, teacher professional development intervention aimed at improving early adolescent school adjustment. Teachers in rural schools in a Northern Plains state took part in professional development activities across a year. Following a randomized control trial design, Native American and White students' (N = 165) social, behavioral, and academic adjustment was assessed in intervention compared with control schools. Regression analyses, controlling for baseline and demographic variables, indicated that students in intervention schools improved in achievement, and improved or sustained beginning-of-year schooling dispositions, and perceptions of the school social/affective context, in comparison with evident declines for students in control schools. Native American students more so than White students evidenced particular gains in achievement and perceptions of the school social/affective context. (Contains 4 tables, 4 figures, and 1 footnote.)