The Wall Street Journal reported recently that limos have been replaced by double-decker party busses as the prom night transportation of choice. This news may help explain why the average prom now costs American teenagers (and their families) over a thousand dollars, up $200 since 2011. The teenage rite of passage is now being marketed as “the new wedding,” notes Time magazine’s Moneyland blog. Proms are glitziest in the Northeast, compared to less extravagant affairs in the South and Midwest. Yet data gathered by VISA show that it is often families in the poorest financial condition that spend the most lavishly on proms.

Photographer Mary Ellen Mark’s new book Prom (Getty Publications) puts this kind of economic and cultural information on full display. But photography being photography, and Mary Ellen Mark being Mary Elle Mark, there is much more to see and muse over. The book, and an accompanying film made by Mark’s husband, Martin Bell, do more than document the details of prom night—the girls in plunging necklines and the boys in ill-fitting tuxedos, the corsages and crowns. In her photographs these high school students are seen slipping out of adolescence and into the accoutrements of adulthood. There is poignancy in the way they pose, or posture, before Mark and the imposing Polaroid 20x24 camera she used for the project. Their faces offer hints about who they are and where they come from, but their futures remain a mystery.

Mark and Bell traveled to 13 proms across the United States from 2006 to 2009, along with a mighty collection of studio lighting equipment, the giant Polaroid camera, and precious stocks of Polapan 20x24 film. “The camera was flown to the various locations,” Mark told me recently, “but the film can’t be flown—the chemicals in it react at high altitude.”

A great deal of the interest in the photos comes from the way Mark’s subjects confront the camera. “I didn’t direct them to do anything in particular,” she said. “People come up with better ideas than I could ever think of.” Cultural and economic differences can be discerned in the dress and body language of Mark’s subjects. Yet ultimately it is the tender similarities as much as the differences in clothes or attitudes that emerge. “All proms have the same spirit,” said Mark. “The individuals are different of course. The kids from expensive private schools and the kids who don’t come from great privilege are different. They’ve had a tougher road. But the spirit of the prom is the same. You really see that more in the film than in the photographs.”

We asked Mark to choose four photographs from the book that tell the story of the American prom in all its cultural variety. She explains how she made the pictures, and why she thinks they work.

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Kerry Murdock and Matthew Costello, Brooklyn, New York, 2007

“I think it’s hysterical that she has a cigarette holder, that she chose that particular object as a mark of sophistication, like a child. And he looks dorky. I like the way his tie is crooked. It’s cute. People do what they do, and it’s usually much better than what you get when you tell them what to do. If I saw people dancing and they were amazing dancers, I would ask them to dance for the pictures. And that would be fine. But I’m not going to ask them to stand on their heads. I don’t want to demean people.

Precious Mingo and Edward Smith, Newark, New Jersey, 2006

“When you’re working with the Polaroid 24x24 you’re taking a picture and making a print at the same time. There is no going into a darkroom or Photoshop to make corrections. Polaroid is the opposite of digital. We had 18 lights on the set—spotlights and softboxes for background and foreground, underlights and fills on the side, scrims and everything else. Each subject was lit differently. Edward had dark skin and a white suit, which are very popular now. So that presented complicated lighting issues. And you’ve got Precious’s legs. That’s part of the picture. The dress was cut to reveal her legs, and she held it up to exaggerate the look. And that’s how I shot it.”

Ashley Conrad and friends, New York City, 2009

I first got interested in proms a few years ago, when I shot a number of magazine stories about the subject. In 2006, I did a story for People magazine about a prom for patients at the Memorial Sloan Kettering Cancer Center in New York. It was an incredible experience. I wanted to include that prom in the book, but the kids who attend it are young—much younger that most prom goers. It just wouldn’t work. Then Martin went in and found Ashley. She is really beautiful. We interviewed her for the film later. She came down to our studio after she finished her chemo. She’s the only person we did that with.

Juan Cerrillo and Alejandra Bocanegra, Houston, 2008

MacArthur High School in Houston is completely made up of kids of Mexican descent. I teach in Mexico—I have for 19 years—so I felt like I was back in Mexico. The kids dressed just like they do in Mexico. He looks a bit like a 1930s gangster. I loved that high school. In many of the pictures, the couples would stand side by side, very formally, with a distance between them. In others they would embrace to touch in some way. I found that fascinating. Juan and Alejandra are sort of in between, with their arms intertwined.