One morning in the mid-1970s, a solemn announcement came over the intercom at Friends Seminary: “Noted person John Lennon is now in the meetinghouse. Walk, don’t run.”

We didn’t run. But we wanted to.

I ended up perched with the rest of my second-grade class on a hard wooden pew in the balcony of our Quaker school’s meetinghouse on East 16th Street in Manhattan. Built in 1860, the meetinghouse was old, dignified and a little creaky; it had absorbed the echoes of abolitionist debates, suffragist meetings and restless kids failing to sit still. That morning, I wasn’t sitting still. We were children, but we knew the Beatles.

And then, suddenly, there he was: John Lennon.

I remember the hush — a collective inhale — and then the whispers. I’m pretty sure Lennon was dressed in black when he entered. That’s how I always remembered him. He soon stood onstage in his wire-rimmed glasses, looking exactly like the face I’d seen staring from album covers. He was right there.

A ripple of laughter broke the tension. I can still hear his voice, his dry jokes, the wry expression when one boy asked about the beautiful woman who’d accompanied him — not Yoko Ono, but someone else. But the words themselves? Gone. Did he talk about music? Politics? Did he sing? Why was he even there?

For years, I clung to the memory like a relic. It was one of those surreal childhood moments that made me wonder if I had imagined it. It was a story I could tell anywhere — When I was in second grade, John Lennon came to my school! My 22-year-old daughter had heard it so many times she could recite it. But recently, when I brought it up, she looked at me skeptically. “Did that happen?”

I was stunned. Of course it happened. Didn’t it? If this had happened today, there would be mounds of evidence: blurry TikTok clips, tagged Instagram posts, shaky iPhone videos capturing every joke. But in the mid-1970s, an event like this could actually fade and disappear.

I called the Friends Seminary alumni office. They had heard of the “legendary” event but had no photos or records to verify it. Strangely, it hadn’t even appeared in the yearbook that year.

“When did this happen again?” the receptionist asked.

“It was 1974,” I said. But even as I answered, I realized I wasn’t totally sure myself. “Wasn’t it?”

A quick plea in a Friends Seminary alumni Facebook group opened the case. Within hours, former students and teachers chimed in, each clutching their faded scraps of memory. A composite portrait started to come into focus, but nothing concrete.

Alice Stern, who is 65 and a retired librarian, remembers how Principal Seegers — a cautious but friendly Quaker with glasses and a full head of gray hair — stood on the stage and read Lennon’s credentials from an index card as though he was a guest from the Board of Education.

Then Lennon said, “OK,” exaggerating his Liverpudlian accent for effect. “Fire away.”

Apparently we did.

A former 10th-grader remembered asking if the song “#9 Dream” had a hidden backward message. The answer was yes.

An ex-middle schooler recalled blurting out: “What does ‘goo goo g’joob’ mean in ‘I Am the Walrus’?” Several people remembered this question. No one remembered Lennon’s answer.

Some swore he played guitar, but that was wishful thinking. Lou Rowan, a long-retired English teacher who is 83 and living in the South of France, told me that Beatles songs had played on a tape recorder before the discussion began, but Lennon waved off requests to perform.

The most mundane answers endured all these decades.

Did he have pets? Yes, two cats: Major, white with black spots, and Minor, a black tuxedo cat.

A lower-school boy even asked how much money he had, and Lennon replied with a smirk: “A whole lot.”

When the assembly was over, Lennon exited the auditorium and was heading to an interview with student reporters when a sixth-grader named David Rauch made his move.

Ignoring faculty warnings, David dashed forward with a ripped notebook page for Lennon to sign. Now 60, he is associate general counsel at Wells Fargo, lives in Hermosa Beach, Calif., and still has the page, 50 years later.

“I got his autograph first, then asked for the woman with him on the same page because I thought it was Yoko.” It wasn’t Yoko. But May Pang, who was Lennon’s girlfriend at the time, signed anyway.

We students didn’t realize it, but this was near the end of Lennon’s infamous “Lost Weekend,” when he and Yoko were separated. She had kicked him out of their Upper West Side apartment at the Dakota, and he spent 18 months publicly boozing around Los Angeles with musicians like Keith Moon and Harry Nilsson. Soon after his appearance at the meetinghouse, he would return to Yoko, and almost exactly nine months later, Sean Lennon would be born.

May Pang, who was in her early 20s then, had started as John and Yoko’s assistant but, at Yoko’s urging — some say orchestration — had become his companion during the couple’s separation.

Hoping for clarity, I called Ms. Pang, a retired music executive who lives in Forest Hills, Queens, and is 74 now. She remembered the assembly well, but she ruled out 1973 as too early. She recalls her time with Lennon and was fairly certain the visit to the school was in 1974.

“It was for Rick, some kind of makeup for a school event we missed when we were out in L.A.,” she told me, and confirmed that Lennon did not bring his guitar or sing. “Rick” was Rick Sklar, the longtime program director for WABC radio and an early Beatlemania champion. He was also the father of two Friends students and a member of the P.T.A. Through his connections, pop stars like Patti LaBelle, Harry Chapin and even Alice Cooper made house calls for assemblies and school fair concerts. Lennon’s appearance was his latest production.

“Let me know if you get the exact date,” Ms. Pang said. “I’m so curious now.”

I thought that would be unlikely, but then Ms. Stern, the retired librarian, called back with a breakthrough: She had dug through old boxes and found a copy of “Genesis,” the upper school’s sporadically published newspaper.

There he was, on Page 3 of the February 1975 issue (Volume 4, No. 3): John Lennon, peering out from a grainy black-and-white photo in his cap and wire-rimmed glasses.

“Beatlemania returned to Friends Seminary on Friday, January 23, when John Lennon paid a visit to our school,” the article begins. Everybody misremembered the year. Unfortunately, Jan. 23, 1975, was a Thursday, not a Friday. But there was another clue: The photo of Lennon was taken by a student photographer, Christopher Gibbs.

I called Mr. Gibbs, who is 66 and a music professor at Bard College, and told him about my quest. Did he have any other pictures from the day Lennon came to our school? Alas, he didn’t think so. But he said that there was another student photographer there that day, Scott Frances, a senior at the time and the best photographer at Friends Seminary. He had shot for the yearbook and went on to have a long career as an architectural photographer.

Mr. Frances, 66, lives in Sag Harbor on Long Island and is still a working photographer. He remembered photographing Lennon, but it was a traumatic memory. There were no pictures of John Lennon in the Friends Seminary yearbook because he had lost the negatives that very week. He didn’t even have the contact sheets.

“They vanished,” he told me. Fifty years later, he still hasn’t given up the hunt.

“I keep looking,” he said with a rueful laugh.

Then Mr. Gibbs called back, excited. He did find more photos — and his diary.

On Jan. 24, 1975, he had tersely logged his day:

“Saw John Lennon during third period.”

“Most of the questions were very stupid.”

“Listened to some classical music.”

“‘Young Frankenstein’ is in theaters.”

If the diary of a teenage boy can be trusted, we had confirmation of the date that noted person John Lennon appeared at the Friends Seminary meetinghouse.

I called Ms. Pang back with the fleshed-out story.

“Wait!” she said. “Give me the date again?” There was a long pause. Then: “That was probably the last time I was out with him as a couple.”

A few days after the school event, Lennon told her he was going for hypnotherapy to stop smoking — and then he moved back into the Dakota.

She hadn’t seen it coming.

She and John were days away from buying a house in Montauk, she told me. The next item on their shared calendar was meeting with Paul and Linda McCartney in New Orleans. “They had just visited us a week earlier and were going to New Orleans to record,” Ms. Pang said.

Within weeks, the papers reported the news: John was back with Yoko, who soon became pregnant with Sean. From that moment on, Lennon effectively became a recluse, rarely seen in public and never to tour again. May was erased, at least officially. According to her memoir, “Loving John: The Untold Story,” which was published a couple of years after Lennon was shot to death outside the Dakota, they would secretly reconnect at odd times until his death.

Her tone was wistful, with a touch of finality.

“You kids caught him at the last moment of his public life,” she said.