According to court documents, Christopher Belter raped the girl when she was 16 years old. He, too, was a teen, a student at an elite private boys school whose family’s home in western New York was known as a party house where teenagers gathered to consume liquor, marijuana, and Adderall.

It was August 2018, and the girl, identified as M.M. in court filings, was staying at Mr. Belter’s house to spend the night with his sister before heading to Chicago the next day.

According to court documents, he invited her into his room, then threw her onto the bed, pulled her clothes off, and told her to stop being such a baby. She described focusing her attention on the leaves of a plant in the room as the attack continued at a hearing this summer.

Mr. Belter, 20, was sentenced this week for the assault on M.M. as well as sexual assaults on three other teenage girls. Faced with up to eight years in prison, he was instead sentenced to eight years’ probation by a judge who stated that he had “agonized” over the decision.

“I’m not ashamed to say that I actually prayed over what is the appropriate sentence in this case because there was great pain,” said the judge, Matthew J. Murphy III of Niagara County Court in Niagara Falls, N.Y., at Mr. Belter’s sentencing on Tuesday. “There was great harm. There were multiple crimes committed in the case.”

“It appears to me that a sentence involving incarceration or partial incarceration isn’t appropriate,” the judge continued. Mr. Belter, who is required to register as a sex offender, was told that the probation would be “like a sword hanging” over his head for the next eight years. He didn’t elaborate on why the sentence didn’t include prison time.

M.M.’s lawyer, Steven M. Cohen, said late Wednesday that M.M. was “deeply disappointed” by Judge Murphy’s decision.

Mr. Cohen stated, “The truth of what happened and what he did to his victims is far more egregious than the charges he pleaded to.” At the sentencing, he said his client was “tremendously remorseful,” according to newspapers.

According to WKBK, Mr. Belter attempted to express remorse to his victims at the sentencing, saying that “through treatment and reflection, I’ve come to feel deep shame and regret for my actions” and that “none of you deserved to be in this situation.”

According to court documents, the attacks on M.M. and the three other girls — two 16-year-olds and a 15-year-old — took place at Mr. Belter’s home over an 18-month period beginning in February 2017. He was a student at Canisius High School in Buffalo at the time, where he played rugby. The house, described in news reports as being in a wealthy section of Lewiston, N.Y., a small town about 15 minutes from Niagara Falls, was popular among Mr. Belter’s circle due to the lax rules governing what they could do there.

When Mr. Belter was arrested, authorities also charged his mother, Tricia Vacanti, with providing alcohol and marijuana to teenagers at various house parties. Gary Sullo, her husband, and Jessica Long, a friend, were also charged with serving alcohol to minors.

According to reports, Mr. Wydysh stated in court on Tuesday that the case was “not a party house case.” “It was a sexual assault house,” he said. According to court filings, after initially being charged with more serious crimes in the four attacks, Mr. Belter resolved the charges in 2019 by agreeing to plead guilty to third-degree rape, attempted sexual abuse, and two misdemeanors.

Mr. Belter, who was 16 and 17 at the time of his crimes, received an interim sentence of two years-probation from a different judge. If he followed the terms of the sentence, he would be treated as a youthful offender when his final sentence was decided, reducing or even eliminating any potential prison term and allowing him to avoid registering as a sex offender. According to court records, when explaining the sentence at the time, the judge expressed doubts about Mr. Belter’s ability to comply with its terms. Her skepticism proved to be well-founded.

Among other things, the interim sentence prohibited Mr. Belter from viewing pornography, which he later admitted to doing anyway by installing software on his personal computer to circumvent the restriction, according to court records.