When Reynolds Middle School shut down its classrooms for three weeks, it wasn’t because of Covid-19 cases. On Nov. 16, parents of students at school in Troutdale, east of Portland, received a brief email informing them the school would revert to online learning so that district officials could develop “safety protocols” and “social-emotional supports” to deal with disruptive student behavior, including fights. Reynolds students aren’t alone in being stuck at home again. Thousands of schools in dozens of districts across the U.S. have taken previously unscheduled days off or moved back to remote learning for “mental health” reasons. Other schools...