Clip_12The 16-year-old girl sat on the witness stand, her hair pulled back. She tried to speak steadily as she told a jury about the night when a ritual at St. Paul’s — an elite boarding school here — turned into what she described as a shocking sexual assault.

It was just days from graduation in 2014, the season of a campus rite called the “senior salute,” when older students ask younger ones to join them for a walk, a kiss, or more.

The girl was 15 and a freshman at the time. She had agreed to follow a suitor, Owen Labrie, then 18, to the roof of a campus building to which he had a key.

Then, she said, he took her into a dark maintenance room. When they kissed, she did not object. But soon he began to grope her; he bit her chest too, she said, and tried more than once to remove her underwear.

“I said, ‘No, no, no, keep it up here,’ ” said the girl, signaling above her waist. “I tried to be as polite as possible.”

ClipShe described Mr. Labrie “scraping” the inside of her body with his hands. Moments later, she said, he penetrated her, and with both of his hands visible near her head, she added: “It had to be his penis.”

Her voice shook as she described the encounter escalating. “I wanted to not cause a conflict,” she said. It began to hurt, she said, but she did not know what to do: “I felt like I was frozen.”

The second day of testimony in the trial of Mr. Labrie, who has pleaded not guilty to 10 counts, including three of aggravated felonious sexual assault, brought out excruciating detail and intense emotion from the young girl at the center of the case as she gave jurors her account of the encounter on May 30, 2014.

In graphic detail, with a mix of poise, sorrow and anger, she accused Mr. Labrie of violating her trust and her body, while noting that even as what she described as the assault was happening, she worried about offending him or drawing ridicule from other students at St. Paul’s.

She spoke for several hours about Mr. Labrie, but seemed to avoid looking at the young man, who sat as still as a statue through much of the testimony. Often, he seemed to be looking at the table in front of him, holding a pen in his hand, as the jury of 11 men and three women listened.

The courtroom here in New Hampshire’s capital city was packed — standing room only — as Mr. Labrie’s family, the family of the girl, and others strained to hear the testimony. A loud fan inside the courtroom made it hard to hear the accuser; early on, court officials had to turn it off.

It was all a sign of the growing interest in the case against Mr. Labrie, a former star student who earned a scholarship to St. Paul’s and had been headed to Harvard until the young girl came forward with her accusations.

Mr. Labrie’s lawyers deny that he and the girl had sexual intercourse, and he is expected to take the stand in his own defense. Labrie’s lead defense lawyer, argued that their encounter was consensual and more innocent than the girl had portrayed.

Seeking to undermine her credibility — the linchpin for many cases involving accusations of rape — he has pointed to their cordiality before and after the encounter, noting that she had wanted it to remain a secret, and emphasized that the girl chose to accept his invitation to the “senior salute,” which he described as an old tradition that many young students joined with pride.

Still, the case has cast that tradition, and other student rites, under a harsh spotlight as it explores the culture of sex, gender and entitlement at St. Paul’s — which counts ambassadors, senators and prominent authors among its alumni.

Surrounding the details of the episode, prosecutors have said, is the social context for the crimes of which Mr. Labrie is accused: The senior salute, in which, they have suggested, some boys compete to have sexual encounters with as many people as possible. It is one of many rituals at the school that encourage hierarchy, according to alumni, and Mr. Labrie told the police he had been actively engaged in the tradition, trying to “score” and win.

The girl said she had been more skeptical of the senior salute from the start. She said she had initially turned down Mr. Labrie’s proposition, but when he responded with an email partly in French, she wondered if she had been too harsh, and agreed to join him.

“What a golden change of heart,” Mr. Labrie wrote in response.

The girl said she had thought that they might kiss, but nothing more. She was thinking “it would be cool” to go to a new place on campus with “one of the most popular boys,” she said.

They made a plan to meet, then went to the rooftop. “It was a beautiful view,” she said, but Mr. Labrie wanted to go back inside. He brought her into a machine room, opening the door with a special key that prosecutors have said had been used and passed around by older boys seeking privacy.

They kissed. Before long, surrounded by industrial noise, she said she was groped, and bitten.

Crying on the stand here, she described the sex acts she said he performed, saying he spit on her, and called her a tease. “At one point, I was in so much pain that I jerked backwards,” she said.

Still, she said she worried about making a bad impression. She was younger. He was older and popular. The senior salute was a St. Paul’s tradition.

“I didn’t want to come off as an inexperienced little girl,” she said. “I didn’t want him to laugh at me. I didn’t want to offend him.”

Afterward, she said, she felt physical pain and utter confusion, and blamed herself for the events; it took several days for her to tell anyone, in full, what happened.

“I feel like I had objected as much as I felt I could at the time. And other than that I felt so powerless,” she said, adding, “I was telling myself, ‘O.K., that was the right thing to do, you were being respectful.’ ”

She did not want to upset Mr. Labrie, she said, or draw others into a turbulent situation.

“I thought, I’m at St. Paul’s right now, this is graduation weekend, I cannot be dramatic about this,” she said.

The two messaged each other after the encounter, and Mr. Labrie wrote that he wore a condom during part of it. She nevertheless went to a school medical facility for emergency contraception, she said, and, when asked if she had had a consensual sexual encounter, she said yes. “It was so much easier,” she said, noting that at the time she was running late to save her parents seats for her sister’s graduation ceremony. She added, “It’s been really hard to stand up.”

Mr. Carney could seek to highlight that conversation with the nurse during his cross-examination, which is scheduled to continue. He may also delve further into whether the girl’s expectations were as innocent as she suggested.

The girl — who left the courtroom with her hands folded in front of her, guided by the prosecutor, will probably once again be on the stand testifying in great detail for much of the day.