The purpose of this study is to investigate the current situation of the academic achievement gap before and after COVID-19. To this end, three indicators (amount of change in the grade of academic achievement, the Gini coefficient of the distribution of academic achievement, percentage of middle-level academic performance) were calculated using the academic achievement distribution data of 382 middle schools in Seoul, and the differences between groups were identified. As a result of analyzing the percentage of achievement grade between two groups when the school year changed, the achievement gap proved to be widened as both the focus group (8th grade in 2019) and the comparison group (8th grade in 2018) went up from 8th grade to 9th grade. However, the focus group experiencing COVID-19 showed a larger increase in the Gini coefficient and a larger decrease in the percentage of middle-level academic performance than the comparison group. While the highest grade (A) increased when both groups changed from 8th grade to 9th grade, the lowest grade (E) also increased for focus groups experiencing COVID-19, suggesting deepened polarization. Also, even if the trend of changes in the distribution of academic achievement grades in the same grade at same school proved that the gap in academic performance has been steadily increasing since 2018, the degree of the gap was the largest in 2020 when COVID-19 spread. A different aspect of the changes in percentages of grade distribution between comparison period(2018-2019) and focus period(2019-2000) when the increase of grade (E) is especially distinguishable demonstrates the deepened achievement gap.