The increase in urban heat waves has led cities to develop adaptation strategies. In 2018, the city of Paris created the "OASIS" program aimed at turning school and college courtyards into islands of coolness and vectors for raising awareness and fostering social ties. Combining a difference-in-difference strategy and hedonic regressions, we evaluate the causal effects of green schoolyards on housing prices. The results show a substantial increase in prices of dwellings situated within 20m of a green school : three years after the renovation date, prices increase by 14% in these dwellings compared to dwellings located close to a non-green school. We also find that the externality effect of schoolyard renovation decreases steeply with distance. Our results therefore indicate that the greening of existing urban elements can improve living conditions in the surrounding area