As schools resumed classes last week, teachers, students and parents arrived at Rafael Veloz Elementary in Ciudad Juárez wearing white, holding hands and weeping. The school’s head teacher, Elsa Mendoza Márquez, had been killed on Aug. 3 in the mass shooting in El Paso. The parents had to explain to their young children why she was missing. The assistant head told them that Ms. Mendoza would always be in their hearts.