Like a stone tossed into a pond, a single act of violence causes waves of loss in the schoolhouse: Test scores drop. Attendance falls. Instruction time wanes. In the days following a shooting, students often spend more time with therapists than with math teachers. Art classes become therapy sessions. High school pep rallies turn into student-organized memorials for fallen classmates. And students’ academics suffer. When students return to virtual classrooms, the initial focus will be on mental health, D.C. Public Schools Chancellor Lewis D. Ferebee said in an interview. His team hosted a virtual session for families this summer about how children process grief. Activists across the country, meanwhile, are calling on lawmakers to divert money from school security and pour it into mental health and violence prevention programs.