Less than a month into Winston Nguyen’s teaching career at Saint Ann’s, an elite private school in Brooklyn, his eighth-grade students discovered that he was a felon.

While leading an algebra lesson, Mr. Nguyen had shown the class a TikTok video, which led the students, inevitably, to search for him on the internet.

What they found was a startling torrent of headlines from about four years before, when Mr. Nguyen had been charged with siphoning hundreds of thousands of dollars from an older couple he worked for in Manhattan.

That evening, in October 2021, Vincent Tompkins, the head of Saint Ann’s School at the time, emailed class members’ parents, acknowledging the teacher’s criminal conviction and saying they had nothing to worry about.

“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,” Mr. Tompkins wrote.

His promise proved hollow. Within a year Mr. Nguyen, 38, was posing as a teenage boy on Snapchat and soliciting sexual photographs and videos from students at Saint Ann’s and other local schools, according to Eric Gonzalez, the Brooklyn district attorney. This spring, Mr. Nguyen was arrested near the school. Later, he was charged with nearly a dozen felony counts. In court last week, a prosecutor said that a plea deal was in the works.