Indoor air quality (IAQ) in buildings has a significant influence on the health of occupants, both from a physical and psychological point of view. In schools, and due to the complex and diverse activities carried out, indoor air has a direct impact on students’ performance. To improve IAQ one of the strategies is the use of plants, which have the ability to absorb indoor contaminants. This article shows the improvement in IAQ, through the analysis of the carbon dioxide and volatile organic compounds, using plants in an Eco-Campus classroom. Another line of investigation and taking into account the environmental sustainability of water efficiency on buildings and cities, is the analysis of plant growth through watering using treated wastewater (TWW). The results show a clear improvement in air quality through the installation of a natural green structure. Plant development seems not to be harmed using TWW for irrigation.