Students retrieve their cellphones from pouches where they are stored during the school day at Bayside Academy in San Mateo, Calif. Gov. Gavin Newsom on Monday signed a bill into law that will require California school districts to restrict or ban student cellphone use, thrusting the state with the largest K-12 population in the nation into the forefront of a growing movement to get distracted students off their devices in the classroom and focused on learning. The law, called the Phone-Free...