In this study, we investigated the indoor air quality (IAQ) in classrooms with exhaustventilation systems and in naturally ventilated classrooms. In the latter, we found peak CO2-concentrations of more than 4000 ppm. 1500 ppm was exceeded during 40 to 86% ofteaching time, dependent on class size. The windows were opened rarely in winter which ledto low mean air exchange rates of 0.20 0.23 h^-1. The operation of mechanical ventilationsystems improved IAQ considerably. Peak CO2-concentrations decreased to less than 2500ppm. 1500 ppm was exceeded for only 7 to 57% of teaching time. Mean air exchange rates of0.36 0.41 h^-1 were determined. The higher air exchange rates caused no measurable increasein heating demand. With respect to mould problems, no significant amount of viable airbornefungi was found. This didnt change if children were present. The viable airborne bacteriaconcentration increased during the lessons. No difference in viable airborne bacteriaconcentration was found in mechanically and naturally ventilated classrooms as long as thewindows were closed. Results indicate, that wide opened windows in naturally ventilatedclassrooms may increase viable airborne bacteria concentration due to air turbulence, but thisis to be investigated further. The acceptance of the mechanical ventilation systems by theteachers was very good. No problems with draught or noise were reported.