Two couples from Mary Ellen Mark’s exhibit Prom—one from her alma mater, Cheltenham High School in Wyncote... and one from Fontbonne Hall Academy, in Brooklyn, New York

Anyone who after more than a half-century finds herself back at her high-school prom might well be having a nightmare. But when photographer Mary Ellen Mark returned to Cheltenham High School for prom in Wyncote, it was a dream years in the making. “I’ve kept several photographs from my high school days,” Mark has written. “The one that has intrigued me the most is my prom picture.”

From 2006 to 2009, with a rare Polaroid 20x24 Land Camera, Mark photographed revelers at more than a dozen public, private, and parochial school proms nationwide. The Philadelphia Museum of Art will premiere nearly 60 of the resulting black-and-white portraits in a new exhibit, Prom, which opened July 1 and will be on display through October.

Documenting a milestone event in the lives of young Americans appealed to Mark, who “wanted to do something optimistic.” For Mark, Prom is also a homecoming of sorts—pun intended. As a child, she visited the Art Museum, which gave her a solo show in 2000; in 1994 she received an honorary doctorate from the University of Pennsylvania; furthermore, Mark dedicated the show’s book of the same name to Philadelphia philanthropist and Art Museum trustee Lynne Honickman and her husband, Harold.

While a 35mm camera is “about capturing a moment in time,” explains Mark, this process with the Polaroid was “about creating one big, beautiful object that shows extraordinary detail, both similarities and differences.” Indeed, the portraits of these couples recall her seminal series Twins, which explored the same detailed duality with identical and fraternal siblings.

Mark observed that for her subjects, life at the cusp of adulthood seemed difficult, whether they went to fancy private schools or tough inner-city ones. “The most moving part of the project was the teenagers themselves—their hopes, their dreams. My own high school, Cheltenham High School, has a diverse student population,” Mark notes in her book’s preface. “While I was photographing a graduating senior who was nine months pregnant, the principal of the school came to the studio to watch the photography in progress; I think he was a bit taken aback.”

Mark notes the striking variance in the socioeconomic circumstances of the students was even more evident in her husband Martin Bell’s related documentary, where interviews with the promgoers on their big night reveal “the advantage of wealth and the confidence of the rich kids,” she says.

Bell’s 33-minute film, part of the exhibit and included with the book, is a work of art in its own right. It brings the promgoers to life by allowing them to articulate—with a winning and often humorous candor, punctuated with sullen bravado and often cracking voices—how they chose their date, their attire, and, as Mark puts it, “how they want to display themselves.” Prom is on view at the Philadelphia Museum of Art’s Perelman Building, 26th Street and Fairmount Avenue, 215-684-7602