CONCORD, N.H. — The prep school graduate accused of raping a younger student at the elite St. Paul’s School dropped his head and sobbed for the first time since the start of his trial: He had been found not guilty on Friday of felony sexual assault charges, but was convicted of having sex with a girl who was below the age of consent.

The accuser sat in the front row, tightly flanked by her family, her father’s hand on her head, her mother’s arm around her shoulders. She cried, too, in the main hall of the drab courthouse here, before stepping out the back door.

So ended the trial of Owen Labrie, 19, and with it a rare exploration of the backslapping sexual culture among some students at one of the nation’s most exclusive boarding schools. Over nearly two weeks, jurors listened to prosecutors and defense lawyers ask witnesses about a custom called the “senior salute,” in which older students at St. Paul’s propositioned younger classmates for a last-chance encounter before graduation.

But at its core, the case was about an intimate encounter last year between a 15-year-old girl and an 18-year-old acquaintance, and whether she consented as it escalated.