This study measured PM2.5 concentrations in 30 schools in Santiago, Chile. Sampling of schools was random with respect to local climate zones (LCZ). Low-cost PM2.5 sensors were installed both indoor and outdoor. The outdoor sensors’ data were highly correlated with the closest regulatory air quality monitoring station (median R2 = 0.62, IQ range: [0.5, 0.71]). Indoor and outdoor PM2.5 levels were highly correlated (median R2 = 0.91, IQ range: [0.76, 0.95]) because of natural ventilation conditions. There was no significant difference between the average PM2.5 in classrooms located far (30 µg/m3) and near (30.4 µg/m3) the street (p = 0.9), suggesting little PM2.5 spatial variability within schools’ boundaries. High classroom infiltration factors (Finf) were estimated (median Finf = 0.9, IQ range: [0.82, 0.98]). Public schools had significantly (p < 0.05) higher indoor concentrations than private ones in each season (57.1 and 39.3 µg/m3 in autumn, 43.1 and 35.2 µg/m3 in winter, 16.9 and 12.6 µg/m3 in spring). In winter season schools in zones with dense midrise buildings (LCZ2) had higher average outdoor PM2.5 concentrations (55.8 µg/m3) than those in zones with open arrangements of midrise buildings (LCZ5) (48.3 µg/m3) — p = 0.06. In spring season, outdoor average concentration was higher for schools in a dense mix of tall buildings (LCZ1) (22.5 µg/m3) in comparison to those in open mid-rise buildings (LCZ5) (16.7 µg/m3) — p = 0.02. Hence, higher building density (LCZ classification) due to limited air circulation promotes higher outdoor and indoor PM2.5 in schools. This should be considered in future urban planning.