A class of first graders scampered among nearby pine and birch trees, each holding a stack of the teacher’s homemade laminated “outdoor math” cards. “Find a stick as big as your foot,” one read. “Gather 50 rocks and acorns and lay them out in groups of ten,” read another. Working in teams, the 7- and 8-year-olds raced to see how quickly they could carry out their tasks.