Green schoolyards can reduce stress, promote resilience, build competence, and promote focusing in students aged 6-18, as shown by this ethnographic study in Maryland and Colorado. The green schoolyards included a two-acre wooded area, a quarter-acre outdoor classroom habitat, and a high-school vegetable gardening program.
Chawla, Louise, Kelly Keena, Illène Pevec, and Emily Stanley. (2014). Green schoolyards as havens from stress and resources for resilience in childhood and adolescence. Health & place, 28, 1-13.