The presence of microorganisms in indoor air represents a dangerous issue with a major impact in the field of health protection and environmental engineering. Low indoor air quality is a major trouble in government schools because of the massive density of students per classroom, inadequate supply of air from outside, as well as weak construction and poor maintenance of school buildings. Evaluation of microbial contamination within the indoor air is important to assess health dangers and establish requirements for tracking indoor air quality. Evaluation of microbial contamination is of notable significance in intensively populated centers as government schools. A study was carried out among a hundred and twenty randomly classrooms of twenty government primary schools of Riyadh city in the academic year 2018/2019. For the determination of microbial pollution, passive air sampling settle plate technique was utilized by exposing a Petri dish of appropriate agar media for one hour. Results showed that the counts of bacteria and fungi were higher in government schools in low socioeconomic districts as compared to those of high socioeconomic districts. The indoor analyses reveal that the concentration of bacteria was higher than counts of fungi. In general, Gram positive microorganisms were the dominant organisms. Gram positive bacteria, Bacillus, Micrococci and Staphylococci were the predominant bacterial strains in the schools under study. Attention has to be paid for controlling the physical factors that assist the growth and reproduction of microbes in indoor air of classrooms to keep the students and teachers healthy.