The ClimACT project has been developed under the priority axis “Low Carbon Economy” of the Interreg SUDOE program. It aims to support the transition to a low carbon economy in schools. Environmental audits addressing energy and water consumptions, waste management, travels to school, procurements and green spaces have been carried out in 38 pilots schools of Portugal, Spain, France and Gibraltar. Indoor air quality and ventilation measurements were also achieved. The concentrations of 9 aldehydes and 10 selected VOCs were measured from passive sampling in two classrooms of each school. In addition, TVOC, CO2, CO, PM2.5 and PM10 concentrations were monitored for a period of 2 days or 1 week in French schools. Of all the individual VOCs and aldehydes investigated, only formaldehyde exhibit some concentrations over the guideline. On the other hand, PM10, PM2.5, CO2 and TVOC concentrations during the class hours are over the guidelines in 64%, 45%, 43% and 20% of classrooms, respectively. The time series of concentrations highlight that a significant part of TVOCs originates from the occupants. Similarly, PM10 concentrations over the guideline are mainly contributed by biogenic emissions and resuspension of settled particles. Another interesting conclusion coming from the study is that there are no significant differences in the characteristics of the air pollution in secondary schools, high schools and universities, when compared to primary schools. However, the characteristics of indoor air pollution in classrooms may be slightly different in Spain, Portugal, Gibraltar and France.