Several studies have found that indoor air quality (IAQ) in schools is often poor and may affect the health of the pupils. Building ventilation is a means to reduce pollutant indoors but different designs should be evaluated for their effectiveness in different environments. In a field experiment performed in four classrooms in one school building we alternately supplied the air according to the mixing and displacement mode and collected information on exposures, pupils’ perception of IAQ and climate, health symptoms and performed clinical examinations. At breathing height, room temperature, relative humidity and the concentration of CO₂ and cat allergen were similar in the periods with each ventilation type. The children perceived indoor air quality as similar in the two ventilation regimes, and there were few differences in symptom reports or clinical parameters. However, the pupils reported more eye symptoms during displacement ventilation.