He was in High School, so likely a minor. "Possible sexual misconduct" could mean practically anything. Misconduct could be that he told some girl she had a nice rack, which, though of late it could ruin your life, in that era, was normal banter. Given the "possible" and all the anonymity, it could also be NOT sexual misconduct of any kind.
On the FBI, the story I read said "Federal investigative authorities". That might be the FBI, or it might be somebody in OPM human resources.
This is another one that smells of "set up" to me.