This longitudinal study examined the cross-lagged relationships of ADHD symptoms, school adjustment and academic performance during the transition from primary to middle school (Grades 6-7) in a Finnish community sample (N = 311). We found that the mechanisms were different for boys and girls: for boys (N = 149) the effect ADHD symptoms had on academic performance mediated via maladaptive achievement strategies, but for girls (N = 162) the effects on lowering Grade 7 academic performance were direct. In addition, ADHD symptoms were associated with SES and pedagogical support only among boys.