This study investigated gender- and age-related differences in academic motivation and classroom behaviour in adolescents. Eight hundred and fifty-five students (415 girls and 440 boys) aged 11-16 ("M" age = 13.96, "SD" = 1.47) filled in a questionnaire that examined student academic motivation and teachers completed a questionnaire reporting student classroom behaviour. Interestingly, early adolescent boys' (11-12 years) self-reported academic motivation was significantly more closely associated with reports of student classroom behaviour completed by teachers. However, a surprising result was the significant drop in girls' adaptive motivation from early to mid-adolescence (13-14 years) and a significant increase in mid-adolescence (13-14 years). Furthermore, teachers reported a significant increase in negative classroom behaviour in mid-adolescent and late adolescent girls (15-16 years). The need to further understand the association between academic motivation and classroom behaviour at different stages in adolescence, and to design interventions to improve classroom behaviour, is deliberated.