A courtroom can become a sort of time machine.

The criminal trial of Paul Geer, a former music teacher, played out in federal court in Albany, N.Y., last week. But testimony and photographic evidence transported everyone back to the 1990s and early 2000s to a town 125 miles away, Hancock, and to the Family Foundation School’s secluded campus in the woods.

The reform school is long closed and has settled several lawsuits by former students accusing Mr. Geer of sexual abuse over decades. But the trial brought the place back into the public spotlight.

There is a photo of a large basement lined with bunks. The female students slept there, beneath Mr. Geer’s home. There is the barn, where he held practice for his young singers.

Middle-age men and women sat in the witness stand and were asked the same question: “Do you see Paul Geer here today?”

They scanned the room, resting their eyes on the stout, bald, bespectacled man hunched at the defense table. Some knew him when he was in his 20s or 30s. Now he is 57.

They all pointed — him.

They were asked about how he terrorized them, or worse, decades ago, when they were teenagers.

In 2024, Mr. Geer was charged with six counts related to bringing three children across state lines to engage in sexual activity. The case led to a trial that began on Feb. 19.

On Friday, closing arguments took place. About a dozen former students who had mostly never met before, having attended the school at different periods, watched in the gallery. The jury began deliberating soon after.

Over the past week, witnesses were shown photographs of the place many have tried to forget. Their names have been redacted from public records in the case, but several have come forward in interviews and lawsuits.

“This is the isolation room where I had to stay for five days,” one former student, Elizabeth Boysick, 41, testified, looking at a picture of a tiny, windowless room. “It’s very hard to look at. Nobody should be treated like this. Especially children.”

The school was founded in the 1980s by Tony and Betty Argiros, a couple who had each struggled with addiction and built the place on the tenets of Alcoholics Anonymous and its 12-step program. Parents from surrounding counties and states sent their troubled children to the small campus, billed as a “therapeutic boarding school,” in the foothills of the Catskills.

Upon arrival, the children were strip-searched in front of other students, and an adult watched as they rubbed lice shampoo into their hair and genitals in a shower, according to testimony. They were assigned to a “family,” with staff members playing the role of parents.

Mr. Geer, who taught at the school from the early ’90s until it closed in 2014, was the “father” of Family Six. He openly described himself to students as a sex addict who hit rock bottom while driving one day and nearly crashing as he masturbated, former students testified. “This constant, returning story,” Steve Zahoroiko, 43, a former student and, later, a marine, testified. “That was the shining moment in his life, when he turned everything around.”

He forced students to admit to impure thoughts and actions in front of their new family. “Talk about whatever sex lives we supposedly had,” Mr. Zahoroiko said.

Mr. Geer was repeatedly described as flying into a rage when confronting a student. “Being screamed at by him very close to my face,” Ms. Boysick testified. “Red-faced, sweating.”

Other students recalled being forced to run in place all day — “trotting” — or haul buckets of rocks up and down a hill as a punishment.

Prosecutors called several former students who said they were sexually abused by Mr. Geer. A 39-year-old man identified as “Victim 3” testified that he had been forced by Mr. Geer to join the choir — “I wasn’t a singer” — and that the teacher had abused him on a school trip to Toronto, in a hotel room. Prosecutors showed videos of his singing in the choir, a younger Mr. Geer energetically conducting the group.

“Never thought I’d be up here on the witness stand talking about this, ever again,” the former student testified. “But he was the devil.”

Mike Milia, 46, another former student, testified that Mr. Geer took him on a fishing and sightseeing trip to Maine for several days in 1994, when he was 15. His parents did not know about the trip. Prosecutors showed photographs of the smiling teenager posing before roadside signs — “Brake for Moose.”

“We never put a fishing rod in the water,” Mr. Milia testified. Instead, Mr. Geer bought beer and pornographic magazines and sexually abused the teen for days, he testified.

Mr. Geer’s lawyers with the federal public defenders’ office sought to soften the grim portrayal of the man and the school. A former administrator, Emmanuel “Mike” Argiros — a son of the founders — testified that he had never heard complaints about Mr. Geer abusing children, and that he had sent three of his own children to the school, in part for its excellent music program.

Former students were confronted with smiling yearbook photographs of their teenage selves, paired with glowing testimonials about their time there. Mr. Zahoroiko, the former marine, chuckled on the witness stand and said the students did not write those blurbs; the school did.

Defense lawyers have raised inconsistencies in the students’ versions of events over the years, in F.B.I. interviews and elsewhere. Elizabeth Ianelli, a former student and an organizer of the earliest public attacks against the school, wrote a memoir of her time there, “I See You, Survivor,” published in 2023.

But she was called as a witness for the defense on Wednesday, and lawyers raised contradictions between what she had written and what she testified.

Throughout, Mr. Geer watched in silence, occasionally wincing in apparent physical pain and seeming to have difficulty rising from his chair when jurors entered or left the courtroom.

Lauren LaCroix, 34, flew to Albany from San Diego, where she lives with her husband and young sons, to watch the trial. She had tried to put her time at the school behind her. Now, she found comfort in meeting other former students.

“There’s no explaining,” she said. “They get it right away.”

Listening to accounts of abuse, she thought of a female staff member who had pulled her aside shortly after her arrival at the school. “She said, ‘I never want you alone with Paul Geer,’” Ms. LaCroix recalled.

On Monday, after an afternoon and a following morning of deliberations, the jurors returned with its verdict.

Guilty on the two counts involving the choir singer in Toronto.

Guilty on the two counts for Mr. Milia on the road trip to Maine.

But jurors were unable to reach a verdict on the counts related to Ms. Boysick — the only counts without photographs to document her time with Mr. Geer.

To those in the audience, Mr. Geer appeared to tear up with emotion at the verdict. He was led away in handcuffs, to remain in jail until his sentencing in July.

Ms. Boysick, who was among the first students to publicly come forward, using her name in a lawsuit, felt vindication, not loss, in the verdict, despite the outcome of the counts involving her.

“I’m going to totally own those guilty verdicts,” she said. “It wouldn’t have happened if it wasn’t for me.”

She said that moments before the verdict, Mr. Geer looked at her — “Wide-eyed, pure fear, what’s about to happen to me?” she said.

It was a feeling Ms. Boysick and her old classmates in the gallery once knew well.