Schools are moving to ban smart phones in class. Research suggests it’s the right move.

Psychologist Jonathan Haidt’s new book, “The Anxious Generation,” lays out the latest research on what constant screen time is doing to America’s youth. And the trends are very troubling. Haidt shows how mental health problems, sleep deprivation, social isolation, suicide attempts, and attention fragmentation have all spiked among youth and suggests a direct link to...