"On a plot designated by officials for the open-air school, builders installed a pavilion, gardens, activity areas and open sheds, some fitted with tables and benches for lessons."

"The school grounds also included a larger shed for meals, an enclosed shelter for rainy days and rest periods, a teacher’s room, a kitchen, toilets and a 'cure gallery,' a special structure designed to maximize sun exposure. In a departure from prevailing norms and in keeping with the goals of progressive educators, boys and girls were never separated. Whereas the average school in Prussia—Germany’s largest and most populous state—counted two square meters per pupil, students at Charlottenburg’s forest school enjoyed 40.... The forest school ensured a steady supply of fresh air to the children of workers. Half the school’s teachers were former patients at sanatoria, where they had already recovered from tuberculosis....  By 1908... the first outdoor school opened in the United States, in Providence, Rhode Island, in the dead of winter no less....
"[T]he Providence Open-Air School was housed in an old school building, where a brick wall had been removed and replaced with large windows that always remained open. To protect the school’s 25 'delicate children' from the cold, wool mittens, hats, overshoes and 'sitting-out bags,' the equivalent of today’s sleeping bags, were provided.... For American educators scrambling to meet the challenges of skyrocketing enrollments—the result of rapid urbanization, immigration and the enforcement of compulsory schooling laws—the outdoor schools promised some relief. At least it would remove at-risk children 'from what many health experts considered the overheated and noxious atmosphere of the typical school room,' writes Richard Meckel, a professor of American Studies at Brown University, in an article on the early history of the schools, 'and provide them with sustained exposure to cold air, which was widely believed to promote strength and vigor by stimulating the appetite and increasing respiratory and vascular activity.' It was this line of thinking that drew support from the eugenics movement. 'Eugenicists prioritized the wider society and future generations,' says Weindling, 'and many thought that promoting fitness could prevent infections, which justified open-air schools.'"

From "When Tuberculosis Struck the World, Schools Went Outside/A century ago, a deadly disease sparked a novel concept: teaching in the great outdoors to keep kids safe" (Smithsonian).