When one of New York City’s most prestigious private schools hired Winston Nguyen in 2020, administrators knew about the felony conviction for fraud in his troubled past. But the second chance they offered him backfired. Nearly four years later, Mr. Nguyen, a math teacher, was arrested again, accused of preying on students. And the school, Saint Ann’s in Brooklyn, faced a roiling crisis.

On Monday, Mr. Nguyen, 38, pleaded guilty to a felony and several misdemeanors after being charged with soliciting lewd images and videos from students. When he is sentenced later this month, he faces a possible seven-year prison term.

Mr. Nguyen was taken into custody after the hearing and will be held temporarily at the Rikers Island jail complex.

His plea marks the latest chapter in a scandal that has marred the reputation of Saint Ann’s School and the administrators who hired him.

This is the second time Mr. Nguyen has been convicted of a felony. In 2019, he pleaded guilty to grand larceny and other charges after he was accused of stealing more than $300,000 from his employers, an older couple he worked for as a home health aide.

He served four months at Rikers, and about a year later was hired by Saint Ann’s, a school that charges about $60,000 per year in tuition and caters to New York’s wealthy creative class.

A nattily dressed figure who arrived at class often in a suit and sometimes with a bow tie, Mr. Nguyen transformed a felony record from a liability into a résumé-builder at a school known for embracing unconventional educators. He taught a seminar called “Crime and Punishment” and quickly become a fixture at the school.

It was the kind of opportunity that few felons get.

In interviews with The New York Times last week, Mr. Nguyen tried to make sense of how he squandered it all, and how he plummeted from the promise of his youth — a driven high school student, he was once honored by the mayor of Houston, his hometown, and went on to attend Columbia University — to the reality of being a 38-year-old man headed to prison for the second time in six years.

“I’ve hurt so many people,” he said.

Mr. Nguyen declined to directly address the students he targeted — he will do so when he is sentenced, he said — but expressed remorse for the damage he caused to the school. “It was an incredibly great community to me, and I really, really regret that my actions have painted them in a horrible light,” he said.

Neither the students targeted by Mr. Nguyen nor their families have spoken publicly, and prosecutors have protected their privacy through the legal process.

Sitting in the courtyard outside his Harlem apartment, Mr. Nguyen vacillated between teary recognition of his transgressions and occasional intense bursts of self-analysis. He said he suffers from a mental illness, bipolar II disorder, which he said went untreated during the Covid-19 pandemic, and that he experienced sexual abuse as a child, but he did not make excuses for his behavior. “I very much take responsibility for my actions,” he said. “I made bad decisions.”

In Brooklyn criminal court on Monday, Mr. Nguyen arrived 30 minutes late, wearing an untucked T-shirt, casual slacks and a parka. He carried a large red shopping bag and a large red book, The New Oxford Annotated Bible.

Mr. Nguyen agreed to plead guilty to one count of using a child in a sexual performance and five separate counts, representing five children, of “knowingly acting in a manner likely to be injurious to the physical, mental or moral welfare of a child less than 17 years.”

Daniel Newcombe, an assistant district attorney, informed the judge of the recommended punishment: seven years in prison, 10 years supervision after his release and a requirement that he register as a sex offender for 20 years.

Mr. Nguyen’s sentencing will take place in two weeks.

The judge, Philip V. Tisne, asked Mr. Nguyen if he understood that after the completion of this sentence, should he be convicted of a felony a third time, he would automatically be sentenced to 25 years to life in prison.

“Yes, your honor,” Mr. Nguyen said.

Eric Gonzalez, the Brooklyn district attorney, said the plea agreement held Mr. Nguyen responsible for his “disturbing and predatory conduct” while sparing victims from having to testify.

When the hearing ended, court officers cuffed Mr. Nguyen’s hands behind his back. He slung his head downward as his lawyer, Frank Rothman, patted his back. He was led out a side door.

Outside the courtroom, Mr. Rothman was circled by reporters. “There is no defense one can proffer when you have images on your phone,” he said.

“I don’t know what his first night is going to be like,” Mr. Rothman said. “I’m sure he is anxious. He is going to jail as a sex offender.”

Mr. Nguyen was hired as an administrative aide at Saint Ann’s in the summer of 2020. He had alerted the administrator who interviewed him that he had been convicted of a felony, and at least one Saint Ann’s employee urged the school’s leaders not to hire him.

He quickly became an indispensable member of the staff, helping to manage logistics during the pandemic as he integrated himself into the school community.

The school promoted Mr. Nguyen to math teacher in the fall of 2021 but did not alert parents to his criminal record until after students discovered news stories about him on the internet. In October of 2021, Vince Tompkins, then the head of the school, sent parents an email about the new math teacher. “I can assure you that as with any teacher we hire, we are confident in Winston’s ability and fitness to educate and care for our students,” he wrote.

Within a year, students at Saint Ann’s and other Brooklyn private schools — some as young as 13 — began to receive solicitations via Snapchat for lewd photos and videos. The user behind the anonymous Snapchat accounts sent one student a graphic video of a 16-year-old boy masturbating.

By February 2024, Saint Ann’s had been notified by the Brooklyn district attorney’s office that it was investigating the continued targeting of its students by anonymous Snapchat accounts seeking sexual photographs and videos. School administrators did not notify parents.

Days before the end of the school year, Mr. Nguyen was arrested near Saint Ann’s. He was charged in July with 11 felony counts, including using a child in a sexual performance, promoting a sexual performance by a child and disseminating indecent material to a minor.

The news shocked parents and students and led to a torrent of media coverage.

In December, Saint Ann’s released the findings of an investigation conducted by lawyers commissioned by the school’s board to determine how the school had come to employ a felon.

The blistering report said that the school administration had “shamed” parents who expressed concern about Mr. Nguyen’s background and had suggested they were not in step with the school’s progressive values.

“In some instances,” the report said, administrators “prioritized teachers including Nguyen over the concerns of students and their families about the teacher’s background or behavior.”

In the months since his arrest, Mr. Nguyen mostly has been confined to his apartment. He takes part in video therapy sessions, including group sessions with other people accused of sexual offenses, and has attended occasional church services. Otherwise, he has remained isolated, reading and watching television.

His sister recently visited him from Houston to help him clean out his apartment as he prepares to grow into middle age in prison. “I don’t deserve the family that I have,” Mr. Nguyen said.

In recent weeks he has been culling his belongings. While packing, he came across a warm coat given to him during a cold winter by Bernard Stoll, the man he worked for and stole from before his employment at Saint Ann’s. “People have been very, incredibly good to me, and I betrayed their trust in a very deep way,” he said.

He did not wish to stand trial, he said.

“I am at a place where I know what I’ve done,” he said. “I think part of the reason I feel so horribly is I just don’t know any way I can make it better for the kids, or for their families or for the school. I accept this sentence because I know that I did something wrong and I want to answer for it.”