When the pandemic COVID -19 led to school closures, many of us had no idea that this disruption would last months and perhaps more than a year. Curriculum-based assessment (CBA) is an assessment process that uses academic knowledge picked directly from the material taught in the classroom. This form of criterion-referenced assessment connects evaluation with instructional programs by informing teachers of student progress and learning challenges. In this study a nationwide exam called Transition to Secondary School Exam TEOG Exam was used as a curriculum based tool since TEOG Exam is also second or third exam for semester. The data showed a one and a half year learning loss in both Turkish and mathematics based on Woesmann's (2016) criteria. The difference in the scores for math exam between 2016 and 2020 was 10.32 points which accounts for half of exam standard deviation. Also, the difference in the scores for Turkish exam 2016 and 2020 was 10.91 points which is also around half of the standard deviation. Additionally, based on gender there was a statistically significant decrease of 12.04 points in the mathematics scores of girls and 8.43 points for boys showing a 15-month learning loss for girls. and a year of learning loss for boys. These results showed that girls had more learning losses due to COVID-19 pandemic. According to the mother's education level; the decrease for mathematics scores were between 9.73 (elementary school degree) and 22.02 points (associate degree). This finding shows that the learning loss in math based on mother educational level is between 15-months and 2.5 years. Similar findings showed up for Turkish scores as well. Turkish scores decreased between 10.43 (elementary school degree) and 22.24 points (associate degree). This finding shows that the learning loss in Turkish based on mother educational level is between 15-months and 2.5 years. These results show that some students did not learn new material after the outbreak and even slipped backwards. Ministry of National Education should take important steps to prevent the effects of COVID-19 pandemic and minimize and resolve the learning losses emphasized in this study.