To investigate school performance of international adoptees in relation to their cognitive competence. From the population of all male Swedish residents born 1973–1976, registered in the census 1985 and with complete test scores from military conscription, the following study groups were identified: Korean adoptees (n = 320), non-Korean adoptees (n = 1,125), siblings (children born by adoptive parents, n = 190) and Swedish majority comparisons (n = 142,024). Global scores from intelligence tests at conscription were compared with grade points from the last compulsory school year (year 9). Linear and logistic regression was applied in statistical analyses. The mean grade points in theoretical subjects were lower in non-Korean adoptees than in the majority population, but when global test scores from military conscription were adjusted for, outcomes were significantly better, equal for physics, than in the majority population. The grade points of Korean adoptees were higher than in the majority population and the same held true after adjusting for global test scores. When SES was taken into account, the risk of poor school performance (only completed lower subject levels) increased in non-Korean adoptees compared to models only adjusted for age and sex. Male international adoptees generally perform better in school than expected by their cognitive competence. A cognitive evaluation is important in the assessment of adoptees with learning difficulties.