Sofia Campoamor sings with the Whiffenpoofs at Politics and Prose Bookstore. (Sarah L. Voisin/The Washington Post)

When Sofia Campoamor was a ninth-grader at Washington’s Sidwell Friends School, a special-guest group dropped by her chorus class: the Whiffenpoofs, the oldest college a cappella outfit in the country. The members told Campoamor and her friends about their astounding opportunity — a one-year trip around the world as members of a storied Yale University troupe that performs for packed concert halls, television audiences and presidents. Cole Porter had even been a member in 1913.

“I thought it was so amazing,” Campoamor recalls. “But it was never something I thought I would be able to be part of.”

All 14 people visiting her high school classroom were male — just as every member of the Whiffenpoofs had been, dating to 1909.

Every member, that is, until Campoamor.

This week, seven years later, Campoamor stood in that same chorus room, talking to students who were once in her shoes. And her T-shirt told them what she had become, and what they could someday become, too: a Yale Whiffenpoof.

It was a triumphant moment during a year-long tour in which Campoamor, 21, has become an a cappella icon.

The Whiffenpoofs, a group of 14 students with powerful pipes selected each year when they are juniors at Yale, are unique among collegiate a cappella groups, and not just because of their oldest-in-the-nation status. Once the singers are tapped as Yale juniors, they all take a year off from college. They spend that year touring the country and the world, funding their far-flung travels through sales of concert tickets and CDs, as well as donations from fans.

It’s an opportunity that no Yale women had, even though some auditioned for the Whiffenpoofs for several years as a form of protest. Until last year, that is, when the group made a major decision: It would admit students based on their voice part, not their gender.

That still means only tenors, baritones and basses are in the Whiffenpoofs, not the sopranos and altos that women usually are. But Campoamor, who sang soprano and alto in her first three years at Yale in a co-ed group, made it into the group as a tenor in 2018. She sings mostly high parts that were written in the arrangements for a man to sing in falsetto.

“Taking the gender as a requirement out of the equation helps open up who can participate,” she said on Thursday as she prepared for a concert at Washington’s Politics and Prose Bookstore, a few minutes from her childhood home in Cleveland Park. “I feel so grateful. And also it makes me sad sometimes, for all of the people who should have been able to be considered for this also.”

Campoamor says her fellow singers never make her feel different when they’re hanging out together on tour, and she blends in well enough onstage, accentuating the Whiffenpoofs T-shirt, or the far fancier uniform of white tie and tails, with big hoop earrings set off against her long hair. But at concert after concert, Campoamor is a magnet for attention.

At Politics and Prose, store owner Lissa Muscatine started the event by pointing out to the crowd packing the aisles: “You can tell if you’ve read the news, and by looking at them, there’s a significant, historic difference in this year’s Whiffenpoofs.” The audience — mostly retirees at this 4 p.m. weekday concert — whooped and cheered, especially the women.

As soon as the concert ended, older women rushed up to meet her. “Good for you!” one said, shaking her hand. “The first woman! That’s just fabulous!” said the next.

“Congratulations,” one fan said. “I’m Yale ’81. My father was a Whiffenpoof. I’ve been watching you — well done. What a journey you’re on.”

The university first admitted women as undergraduates in 1969.

The Whiffenpoofs still have a long way to go, in Campoamor’s opinion. A music major at Yale who hopes to make a living after graduation as a professional composer and songwriter, Campoamor has been paying close attention to the works that the Whiffenpoofs choose to sing, and she has urged them to pick more choices from female artists. In a parents’ weekend concert at Yale this fall, she got Joni Mitchell’s “Both Sides Now” onto the program and proudly sang the solo.

And she’s not sure the compromise of letting women audition as long as they can sing a traditionally male vocal part goes far enough. “Most people who are going to be able to sing these parts, it’s still primarily going to be an opportunity for men,” she said, emphasizing that such a unique experience as a trip around the world shouldn’t be off-limits to most Yale women. “To me, the fact that this is still most of the time going to be going to people that are men means that we need to do some more thinking about how to create an ideal a cappella system for our seniors. … I don’t think we’ve finished.”

Next year’s 14 Whiffenpoofs, selected this week to replace the current class, also include one woman.

At the Politics and Prose concert on Thursday, Campoamor dug deep down in her vocal range to croon Elvis Presley’s “Can’t Help Falling in Love” as a soloist. After starting the verse with a husky “As the river flows,” she soared high when she hit the chorus — “So take my hand” — with a dozen men harmonizing behind her.

She snapped and shoo-wopped with the group through a selection drawn from many of the Whiffenpoofs’ 11 decades, from alumnus Porter’s “Anything Goes” to Kermit the Frog’s “Rainbow Connection.” Soon, she’ll be singing these songs across the globe. She’s particularly looking forward to Puerto Rico, where she has family, and New Zealand, where she hopes to see landscapes reminiscent of “Lord of the Rings.”

And when the group stops back at Yale, she’ll sing at Mory’s Temple Bar, the pub the Whiffenpoofs have frequented since 1909. She’ll be part of all the old traditions. The cup they all drink from that must never be set down on the table. The closing song at every concert, with its incomprehensible lyrics and references to sheep, that Campoamor now gets to step forward at the concert’s end to invite the white-haired men from decades ago to come up and join. She’ll do it four times more in the Washington area on this tour stop; at each concert this weekend, men will tell her that they had the experience she’s having now, decades ago, and women will tell her they wish they could have.

When she heard about all this in high school, she never thought it could be for her. “I just kind of accepted it. It made me sad, but it didn’t make me angry,” she said. She wishes she had known as a teen that she could change the tradition. “That’s what makes me the most angry now, that I didn’t even think to be angry then.”