The Enforcement Decree of the Elementary and Secondary School Act (2015) mandates that from 2016 all middle schools in Korea must administer one semester as a free semester (FS), called an exam-free semester or, alternatively, a free learning semester. Consequently, some Korean parents and students have expressed a feeling of anxiety about having no exams during the FS. This anxiety arises primarily from their concern about academic loss and neglect of study in the absence of exams, with ensuing lower achievement in future tests. There are many studies showing a positive effect of the FS on learning, but little is known about its practical effect on student achievement. Thus, this study explored if the FS affects students' achievement by estimating the average population treatment effect of the FS policy on academic achievement in Seoul schools. Based on the inverse probability of treatment weight (IPTW) approach with existing empirical data, the study found that, overall, no substantial differences were noticed in the academic achievement between the pilot FS schools and the non-FS schools. The findings provide empirical evidence for the effect of the FS on academic achievement, as well as critical information for policy makers.