Tonight the star prep school student accused of rape. The jury must decide, was it a calculated sexual conquest? Was it consensual? Was there sex at all in the 19-year-old once considered one of the... See More
Tonight the star prep school student accused of rape. The jury must decide, was it a calculated sexual conquest? Was it consensual? Was there sex at all in the 19-year-old once considered one of the best and brightest in his class now facing the prospect of prison. Here's ABC's geo Benitez. This someone saw how vulnerable a 15-year-old freshman was and took advantage of it. This someone was going to get what he wanted. Reporter: Tonight the fate of 19-year-old Owen Labrie is in the hands of a jury in a high-profile case of he said, she said, on the campus of one of the country's most elite prep schools. We started to kiss. Just a little bit. Anyway, she giggled, I giggled. Reporter: It was a make or break decision to take the stand, testifying in his own defense, accused of raping a 15-year-old schoolmate last year. Their encounter a so-called senior salute where a graduating senior spends time with a younger student, sometimes intimately. Why did you send her a senior salute on the eve of your leaving the high school? I wanted to ask her out. Did you want to get together with her? Yeah. Reporter: With his accuser sitting in the front row watching him, Labrie describes taking her to the school's math and science building, seen here in a St. Paul's school promotional video, and laying down a blanket together. The situation got more intimate. Did she indicate at any time that she was uncomfortable? No. Reporter: Labrie says they were kissing and rolling around but he had a change of heart and decided not to have sex with her. I thought to myself, you know, maybe we shouldn't do this. It hadn't been my intention going into the night to have sex. Reporter: The prosecutor wasn't buying you want all these people here to believe that after all that time spent thinking about her and having foreplay, that you just stopped? I didn't just stop. We kissed more afterwards. But I didn't have sex with her. Reporter: Labrie admits he bragged to friends that he did have sex with the girl. He told me that he had sex with . Reporter: Saying in one Facebook message he used every trick in the book. It was a joke. But today it's a lie? Yes, it is a lie. You know. I wanted to boast to my friends afterwards. Reporter: A lie, he says, part of 119 Facebook messages that he deleted from his account after police first contacted him. What's interesting here is the whole defense rests on the theory, the assertion, that Owen Labrie did not have sex, consensual or forced, with the girl. If that can be disproved the whole defense fails. Reporter: His accuser, now 16, took the stand has it week giving her account of what happened the night she says ended in rape. We're disguising her voice due to the nature of the alleged crime. He took your pants off? Right? Yes, I did. You helped him do that, didn't you? I lifted my hips up to make it easier for him, yes. Reporter: Labrie's accuser said she had initially turned down his invitation that night, then agreed on one condition. Yes, only if it's our little secret. What did you mean by that, only if it's our little secret? I didn't want him boasting to his friends. Reporter: So she went to meet him but says at the moment of truth that she wanted him to stop. I didn't kick or scream or -- really push. But I did say no. I said no three times. Reporter: She broke down in court when challenged by Labrie's attorney. Why were you cloudy? I was raped. I was violated in so many ways. Consent is always tricky in cases like this. But here you have a freshman girl and a senior boy. You have a young girl who's looking up to this boy, who wants his attention, who sees him as somewhat socially superior to her. So is she really capable of giving consent when there's such a power differential between them? Reporter: The details of the testimony, critical. Raising questions not only about consent but about the culture at the $50,000 a year prep school they both attended. Labrie there as a harvard-bound scholarship student. His friend taking the stand to describe what other students typically do during a senior salute. They would get together and they could, you know, spend some time together that could involve kissing or more than that sexually. But not necessarily. Reporter: Prosecutors alleging Labrie told his friends he wanted to "Slay his accuser." The word slay could mean anything between kissing and having sex, correct? Yes. And you said you at least wanted to slay her, correct? Yeah. As I mentioned. I was fond of . I often talked about my affection with her with my friends. Reporter: Labrie was the only witness for the defense. But the cross examination of the school nurse, who the alleged victim visited two days after the rape, may have helped his case. She told you there had been an encounter? Correct. You asked her specifically if it was consensual? I did. She told you, yes, it was consensual? She did. Does it hurt the prosecution this young girl didn't tell anyone immediately there was a rape? It hurts very, very much. It hurts. The prosecution has to explain that away. Reporter: Another factor in this case, friendly messages sent right after the alleged rape. Labrie writing, you're an angel. She replies, you're quite an angel yourself. Is this someone who was unwilling? The messages between them are arguably the most important evidence for the defense in this case. The defense is basically saying, look at these messages. Are you really going to convict beyond a reasonable doubt a guy who was exchanging messages like this with her? After the fact? Reporter: Labrie's attorney claims the girl made up the rape charge to protect herself when the high school rumors started. She had to make the decision whether it would be her reputation that was going to go into the toilet or Owen's. And she took the easier choice. Reporter: Labrie faces nine criminal counts, three of them felony sexual assault charges, each carrying up to 20 years in prison. There are a lot of charges here. This is not a simple case. And you could end up with a real divide on that jury. Reporter: In closing arguments today Labrie's defense attorney placed blame on the faculty of the elite new Hampshire school for turning a blind eye to the senior salute. The idea that you would wink at a tradition that senior salute represents is shocking it damages children. And in this case, it damaged both And Owen. This isn't the school's fault. This isn't the fault of the culture that's at St. Paul's school. It was the defendant who manipulated that culture to get what he wanted. Reporter: After three and a half hours deliberating, still no verdict from the jury. They head right back to the courthouse in the morning. For "Nightline," I'm geo Benitez in Concord, New Hampshire.
This transcript has been automatically generated and may not be 100% accurate.