It’s a bright, warm Friday afternoon in June at Spring Harbor Academy, a private school in Westbrook, Maine. A faint aroma of cilantro and lime hangs in the air, traces of the guacamole the students made — and promptly devoured — half an hour ago in a cooking class. Sunlight streams in through a window, and three students now sit at separate tables, peacefully absorbed in their tablet computers. Accompanying each adolescent boy in this classroom is a teaching assistant wearing thick arm pads and...