Introduction: During this pandemic, overcrowding in classroom caused by a lack of educational facilities and poor indoor air quality are the main causes of respiratory diseases among children and adolescents. Therefore, it is essential to measure and assess the indoor air quality where children spend extended periods of time such as school. Materials and methods: This study covered four schools with old buildings and four schools with new buildings in Bayanzurkh, Sukhbaatar, Khan-Uul, Chingeltei district of Ulaanbaatar. We collected PM10 and PM2.5, carbon dioxide, air temperature, humidity, and microbiological count from chosen classrooms and compared to the MNS4585:2016 standard. SPSS-24 was used to do statistical analysis on the information gathered during the evaluation. Results and Discussion: The 24-hour average PM2.5 concentration was 64.3 (95% CI: 64.1-64.5) mcg/m3, which was 4.3 times higher than the WHO guideline value and 1.3 times higher than the MNS4585:2016 standard. The 24-hour average PM10 concentration was 85.3 (95 % CI: 85.1-85.6) mcg/m3, which is 1.9 times higher than WHO guideline value. In older school buildings, the 24-hour average PM2.5 concentration was 5.6 times higher than the WHO guideline value and 1.7 times higher than the MNS4585:2016; the average PM10 concentration was 2.8 times higher than the WHO guideline value and 1.3 times higher than the MNS4585:2016. The air temperature and carbon dioxide concentration in classroom was met the MNS4585: 2016. The average relative humidity of all schools is 24.2±6.5%, which is 14-16% lower than the MNS4585: 2016. Conclusion The indoor air quality of the school in new and old buildings was similar poor, therefore a variety of steps are needed to improve it.