The disparities between the scholastic achievements of girls and boys have been attributed to biological and sociological factors. The present study investigated the validity of these explanations in a multi-variable situation similar to field conditions. Achievement scores of 3446 pupils in the 5th through 11th grades, half girls and half boys, were sampled. The study controlled for teacher gender, teacher seniority, teachers' tendency to assign high or low grades, ratings of pupils' behaviour, pupil age, class size, disciplinary areas (humanities or sciences), test type (multiple-choice or open-question) and school size. Findings indicated that most of the variance in achievement was due to teacher gender, whereas the influence of pupil gender was small. These results support sociological theories about gender and cast doubt on biological explanations. In addition, this study of multiple situational variables contributes to clarification of the controversy about the relationship between class size and levels of academic achievement.