International studies, like the PISA study, detect the performance of pupils. Besides the teaching method, other influences are also relevant for their performance. The environment in the classrooms is supposed to play an important role. The environment in the classroom is affected by many different factors. One of them is the room ventilation. The air quality is mainly influenced by the air change in the room. As most of German schools are not equipped with mechanical ventilation systems, window ventilation controlled by the occupant is the main way to maintain healthy, performance-supporting conditions in the classrooms in terms of air quality. For that purpose the window opening behavior of the occupants was measured in one German school in five different classrooms. A permanent data logging was performed in one school in northern Germany. Five classrooms in four different buildings, built between 1900 and 1990, were measured. Every window in each classroom was equipped with a sensor to detect the time and duration of its opening. The classroom characteristics like glazed area, type of shading device, window opening type and thermal storage properties were documented for analysis. Additionally temperature, relative humidity and carbon dioxide concentration in the interior air as well as the exterior climate conditions were measured every minute for half a year. This allows the evaluation of the parameters with biggest influence on the window ventilation by the occupants. First results show, that window opening events occur mainly in the beginning of the lessons, at their very end or in the breaks between the lessons. Exterior climate conditions influence the occupants behavior in so far that the colder the exterior air is, the shorter the windows are opened. Furthermore the carbon dioxide concentration in the classrooms used as a measure of the air quality or bad air quality itself can not be perceived by the occupants. Pupils and teachers therefore do not open windows despite very high concentrations. First assessments indicate that how windows are opened in the examined German school is not always purposeful with respect to health, comfort and energy use.