The present paper presents the results of the energy and environmental evaluation of ten school buildings in the Greater Athens Area. The research included measurements of the indoor air quality, evaluation of the situation of the building envelope, recording of energy and ventilation systems and generally all the systems that influence the energy output of the school buildings. Experimental investigations were performed in ten different schools and the concentration levels of CO2, CO and VOCs were measured. The analysis of the educational buildings included measurements of several environmental parameters such as temperature, relative humidity and air velocity inside each classroom, while ventilation was examined byestimating the air changes using the tracer gas technique. The thermal comfort conditions have been calculated using two different thermal comfort bioclimatic indices developed to be used for indoor spaces. The investigation of the above parameters was assessed in relation to the energy consumption of the school buildings. Finally the energy performance of school buildings was rated using a kmeans clustering technique.