This study evaluates the results of a measurement campaign composed of 37 on-site individual measurements performed during five months in both primary schools and university classrooms, while presenting a methodology that uses dCO2/dt to identify CO2 tendencies that takes into account classroom topology and ventilation. Results show that variables such as their position and the ventilation strategy of the room, often outweigh the sensor specifications, causing spatial discrepancies beyond 100ppm. Along with the results, recommendations for future measurements are discussed in order to avoid non-representative estimations of the CO2 level of indoor spaces.