Children's exposure to air pollution affects both their health and learning skills. Fine and ultrafine particulate matter (PM2.5, PM1), notably issued from traffic sources in urban centers, belong to the most potential harmful health hazards. However their monitoring and the society's awareness on their dangers need to be consolidated. In this study, raising teacher and pupil involvement for air quality improvement in their schools environment is reached through developing a passive monitoring technique (bio-sensors made of tree bark). The experiment was implemented in two urban elementary schools situated close to a main traffic road of the city of Toulouse (South of France). Magnetic properties, carbonaceous fraction measurements, and scanning electronic microscopy (SEM-EDX) investigations were realized both on passive bio-sensors and filters issued from active sampling. We find that traffic is the main PM1 source for both outdoors and indoors at schools. Higher levels of outdoor PM in the school's environments compared to urban background are reached especially in the cold period. The schools proximity to a main traffic source and lack of ventilation are the main causes for observed PM1 accumulation in classrooms. The co-working experiment with educational teams and pupils shows that the use of bio-sensors is a driver for children empowerment to air pollution and therefore represents a potential key tool for the teachers though limiting eco-anxiety. As PM accumulation is observed in many scholar environments across Europe, the proposed methodology is a step toward a better assessment of PM impact on pupil's health and learning skills. Plain Language Summary Children's exposure to air pollution affects both their health and learning skills. Monitoring airborne particles in school environments from a knowledge co-production perspective seems essential to address air pollution in schools. Here we use tree bark and filter samples to monitor air quality in schools. Teachers and pupils are involved in the implementation process. The experiment was set up in two schools in Toulouse (France), close to the busy ring road. Traffic is the main source of air pollution both inside and outside of the classrooms. Quantities of airborne particles are higher during cold periods. The proximity of the highway and poor ventilation in the schools are the main cause of airborne particles accumulation inside classrooms. Collaboration with school staff and students shows that the use of tree has been a driving force for children to embrace the project and understand the context of air pollution, limiting anxiety related to this topic.