The spate of student suicides in and around Las Vegas has pushed the Clark County district, the nation’s fifth largest, toward bringing students back as quickly as possible. By December, 18 students had taken their own lives. Since schools shut their doors in March, an early-warning system that monitors students’ mental health episodes has sent more than 3,100 alerts to district officials, raising alarms about suicidal thoughts, possible self-harm or cries for care. The reminders of pandemic-driven suffering among students in Clark County, Nev., have come in droves.