Energy consumption in buildings, in schools as well, in Serbia and Montenegro is significantly higher than is defined in world’s energy standards. Heating energy is the biggest part of total energy consumption in schools. Energy losses through the windows reach up to 40% of total energy for heating of building that points out the importance of economically usage and saving of energy. This paper presents one-week analysis of temperatures and heat gains for two classrooms with different type of windows. The analysis is performed by Design Builder software. Due to heat losses analyses continual measurements of ambient and outside temperatures with sampling time of one hour have been performed. The measurements were performed in two classrooms; in one classroom, there are old windows with wood frames, whereas in the other there are new PVC windows. The system for temperature measuring was realized based on microcontroller and a digital temperature sensor. Taking into account measured and simulation results, the relevant conclusions were performed regarding energy consumption efficiency for school buildings heating. .