As millions of teenagers return to school this month after their winter breaks, Pinterest chief executive Bill Ready argues that there is one thing they shouldn’t bring to class: their cellphones.Ready is joining a growing movement of parents, educators and regulators calling for “phone-free schools” — a new strategy aimed at fighting cyberbullying, keeping classrooms free of digital distractions and pushing students to bond with their classmates in real life. Ready is scheduled to tout the valu...