The purpose of this study was to assess indoor air quality as well as actual ventilation in renovated school classrooms. A typical naturally ventilated school building was chosen to install different air inlet units in identical classrooms. Later measurements of carbon dioxide concentrations, temperature, relative humidity and air velocity were carried out. Actual measured values were compared with Lithuanian and European standards. This study gives the evaluation of renovating process in the Lithuanian schools, which began in 1997. The aim of the mentioned process was the reduction of energy consumption in these buildings. Generally such works as modernization of heat substations, replacement of windows, additional thermal insulation of walls and roofs were usually performed. However, the proper attention to indoor air quality and ventilation of classrooms was not paid due to the low budget of these projects as well as underestimation of air quality impact on pupils health. The results of indoor air analysis in classrooms show that renewal of ventilation systems must be considered more seriously when planning school renovation process in future.