The study examines if children with deficits in working memory, phonological awareness or counting knowledge profit in academic skills when taught in age mixed schools in grade 1 and 2. Cognitive functions were measured at the beginning of the first grade in age mixed (N= 564) and age homogenous schools (N= 476). Achievement measures were taken during grades 1 to 4. The results indicate interactions between cognitive deficits and mixing: risk children profit from mixed age classes in the language domain, but should be taught in age homogenous classes when having deficits in phonological awareness. Research consequences are discussed.