One afternoon earlier this year, my 11-year-old son was sitting at his laptop and working quietly on his math homework. At least, that’s what he was supposed to be doing. When I glanced at his screen, equations were nowhere to be seen. He was controlling a monster in the midst of battle, casting magic spells to outduel an opposing player. “That’s not your math homework!” I told him. But it was. His fifth-grade-math teacher had told her students to spend time on Prodigy, a site that looks and