This study has investigated the change of the 4th-8th grades science and mathematics course success of 2142 elementary students in low socio-economic status by their grade level and gender via cross-sectional and longitudinal research approaches. Based on the findings of the study, consistent with the recent national and international studies, girls were found to have higher academic success than boys both in science and mathematics. When the changes in the annual science and mathematics course scores are investigated by the grade level, the mean scores of boys were found to decrease with the increasing grade level, whereas no significant change in any direction was observed for the mean scores of girls. Due to this difference, it has been observed that the achievement gap between girls and boys increases as the grade level increases, in favor of girls. Based on the related literature and the results of this study, it has been concluded that the traditional expectations about the gender gap, suggesting the male superiority in elementary science and mathematics courses, are no longer valid in today's classrooms, even in a sample of students in low socioeconomic level.