Most of Australia's classrooms are equipped with operable windows for ventilation, and occupant-controlled air-conditioners that do not provide fresh air. This can result in insufficient ventilation and high indoor CO2 levels, which comes to the detriment of occupant productivity and health. In this paper, we report on the results of a field study in which we captured indoor CO2 levels in five classrooms at a school in Victoria, Australia. The study consisted of three measurement periods: (1) First, CO2 levels were measured pre-COVID pandemic; here, the readings exceeded the recommended ASHRAE threshold of 1,000 ppm for 73% of occupied hours, while levels exceeded 2,000 ppm 37% of the time. (2) Then, we captured data after the teachers had been informed of the poor indoor air quality (IAQ) and the state government introduced COVIDSafe measures; the impact was significant, with now only about 16% of readings above 1,000 ppm and values almost never exceeding 2,000 ppm. (3) Finally, we installed devices that gave the occupants visual feedback when CO2 levels were high, prompting them to open the windows and doors; this further improved the classrooms' IAQ, with now just 13% of readings above 1,000 ppm. Our study suggests that, while relying on uninformed occupants to operate windows can lead to poor IAQ due to insufficient ventilation, it is possible to considerably influence occupant behaviour through education and feedback devices. While these interventions alone did not mitigate the problem of inadequate ventilation entirely, they were sufficient to keep CO2 levels within a generally healthy range. Considering the large energy savings that are possible by foregoing mechanical ventilation, we suggest that natural ventilation is a feasible operation method for school buildings in temperate climates, as long as occupants are informed about the importance of ventilating, by providing them with CO2 feedback devices. [ABSTRACT FROM AUTHOR]