We conducted multiday continuous monitoring of indoor and outdoor particulate matter (PM) in classrooms with fan-assisted natural ventilation (NV) at five primary schools in Singapore. We monitored size-resolved number concentration of PM with diameter 0.3–10 μm at all schools and alveolar deposited surface area concentrations of PM with diameter 0.01–1.0 μm (SA0.01–1.0) at two schools. Results show that, during the monitoring period, schools closer to expressways and in the downtown area had 2–3 times higher outdoor PM0.3–1.0 number concentrations than schools located in suburban areas. Average indoor SA0.01–1.0 was 115–118 μm2 cm−3 during periods of occupancy and 72–87 μm2 cm−3 during unoccupied periods. There were close indoor and outdoor correlations for fine PM during both occupied and unoccupied periods (Pearson’s r = 0.84–1.0) while the correlations for coarse PM were weak during the occupied periods (r = 0.13–0.74). Across all the schools, the size-resolved indoor/outdoor PM ratios (I/O ratios) were 0.81 to 1.58 and 0.61 to 0.95 during occupied and unoccupied periods, respectively, and average infiltration factors were 0.64 to 0.94. Average PM net emission rates, calculated during periods of occupancy in the classrooms, were lower than or in the lower range of emission rates reported in the literature. This study also reveals that indoor fine and submicron PM predominantly come from outdoor sources, while indoor sources associated with occupancy may be important for coarse PM even when the classrooms have high air exchange rates.