The main aim of this study was to investigate the extent to which prior social skills and academic achievement as well as the teachers' gender and the gender ratio of classes explained boys' and girls' teacher-assessed academic achievement in Norwegian, mathematics and English. Data were collected from 153 classes at 27 Norwegian schools in the autumn of 2012 and 2014, including 2,266 first to tenth grade students, and were controlled for age, cultural background, prior social skills and prior academic achievement at the individual level and for class size. Overall, the results showed that previous academic achievement at the classroom level explained variance in boys' and girls' individual academic achievements in English and mathematics. In Norwegian, only the previous academic achievement at a class level was explanatory for girls subsequent individual achievement. The effects of previous academic achievement seemed to be larger for girls than for boys. Gender of teacher seemed to explain some of the variance for girls' academic achievement in Norwegian and English, but not for boys. In addition, at the classroom level, social skills and the gender ratio did not significantly influence boys' and girls' academic achievement.