[Riana] Alexander, a soft-spoken, studious teen, has become a force for change in her community in raising awareness about students’ mental health needs, most critically the need for school officials to intervene when kids are showing signs that they are at risk for suicide. Schools in her district do a good job responding to crises, but she said where they fall down are in areas of prevention and what Alexander calls “postvention” — follow-ups with students after a peer has died....