As the nation heads toward a chaotic back-to-school season, with officials struggling over when to reopen classrooms and how to engage children online, teachers’ unions are playing a powerful role in determining the shape of public education as the coronavirus pandemic continues to rage. Teachers in many districts are fighting for longer school closures, stronger safety requirements and limits on what they are required to do in virtual classrooms, while flooding social media and state capitols with their concerns and threatening to walk off their jobs if key demands are not met. On Tuesday, the nation’s second-largest teachers’ union raised...