In order to achieve good air quality in schools, where children spend ca. 6–8 hours per day, and to find solutions for improving it, indoor pollution has to be accurately evaluated. Children are among the groups most vulnerable to the adverse effects of PM. Our study focuses on the characterization of different fractions of PM (PM1.0, PM2.5, PM10) measured inside and outside the building of a secondary school in Wroclaw, Poland, located in the centre of the city. PM1.0, PM2.5, and PM10 samples, from indoor (I) and outdoor (O) air, were collected using Harvard cascade impactors during the years 2009/2010. The study revealed that on 90% of days in the wintertime, 24h indoor PM2.5 and PM10 concentrations were higher than the WHO air quality guidance values for ambient air. The PM2.5 fraction found inside the school in winter and the PM10 fraction both in winter and summer resulted mostly from indoor rather than outdoor sources, as shown by the I/O ratio of 1.2 for 24-hour mean of PM2.5 and 1.3, and 2.0 for 24-hour mean of PM10 in winter and summer, respectively. However, PM2.5 and PM10 concentrations during teaching hours (8-hour mean) were much higher than outdoors (2.0 and 2.5 times in winter and 4.1 and 5.6 times in summer, respectively). PM1 concentrations reached comparable levels indoors and outdoors. These data can be considered as preliminary; however, they may act as a stimulus for future studies aiming to estimate the effect of indoor PM concentrations on the respiratory system in schoolchildren