Many schools in Sweden lack a proper indoor environment due to, e.g., poor thermal-envelope properties, overcrowded classes, poor visual appearance and insufficient ventilation. This study aims to explore the integration of a large number of indoor green plants into classrooms’ environments. This case study consists of three parts: measurements of the indoor environment including a final energy model, a questionnaire to the pupils with questions about their well-being and qualitative interviews with teachers. The case was two classrooms in a secondary education facility in central Sweden with an average annual temperature of 3 °C and a long and dark winter period with snow. The results showed 10% lower CO2 and slightly higher and more stable temperatures due to the green plants. Worries about climate change and war among the pupils decreased after several months with the plants and worry about infectious disease increased. The teachers experienced fresher air from the plants and used the plant stands for a flexible classroom design. The conclusion is that indoor plants have the potential to contribute to a better indoor environment, but due to the high number of uncontrolled variables (including the effect of COVID-19) in measurements of real-life conditions, more studies are needed.