In 2006, the School District of Philadelphia, in partnership with Microsoft, opened the School of the Future. The idea was simple enough: Establish a learning environment centered on technology—no textbooks, just laptops and Wi-Fi—that would provide students in relatively poor districts the same benefits that those in wealthier areas enjoyed. The district built a handsome, well-lit building and filled it with state-of-the-art trappings including electronic lockers and Italian-marble bathrooms. It was heralded as a path-defining achievement for public-private partnerships in education. Two years later, Michael Gottfried, now an economist at the University of Pennsylvania but then a graduate student there, was part of a team examining whether such a technological revolution actually made a difference in student achievement. But he soon realized that the technology was somewhat beside the point: “We were talking to a teacher [at the School of the Future] and she said, ‘Here’s the thing, we can talk all you want about smart boards and laptops per student and curriculum moving online, but I have a bigger problem: Half of my class …