In May 1974, Rep. William S. Cohen, a Republican freshman on the House Judiciary Committee, was gathering evidence that led to his eventual vote to impeach President Richard M. Nixon. But outside the Watergate hearings room, Cohen needed a French speaker to respond to all the partisan mail from his French Canadian constituents in Maine.

He was in luck. A French exchange student — a senior at the private Holton-Arms School in Bethesda — had just begun interning in his office that month.

The internship formed an important part of the teenager’s year in America and would ultimately furnish an only-in-Washington coincidence: The congressman-and-future U.S. defense secretary was passing off scut work to the future head of the International Monetary Fund, Christine Lagarde.

“Everyone was writing to the congressman, saying, ‘Impeach Nixon.’ ‘Don’t impeach Nixon.’ So I was introduced to the art of dealing with constituent members,” Lagarde, 55, recalls with a laugh. “During that year, at Holton-Arms, with my host family and interning in Washington, I learned more, and it mattered more to me, probably, than any year of my life.”

To most Americans, Lagarde might be an unfamiliar face that burst into the news as the successor to Dominique Strauss-Kahn, the former IMF managing director charged with the attempted rape of a New York hotel maid. To politicians and economists, Lagarde is the former French finance minister, who, as the new IMF chief, is overseeing the financial bailouts of three Western European countries.

But to a group of women from the Holton-Arms Class of 1974 and to the family that hosted her here, Lagarde was Christine Lallouette. She was the girl who already had graduated from a high school in France but had chosen to spend a year at Holton-Arms as an American Field Service exchange student. She was the girl who liked putting chamomile in her shampoo to make her hair more blond.

She smoked. She was a bit daring.

And she was the chosen confidante of a small clique of girls who didn’t mesh well with the preppier scene. Even today, they recall her impact on their lives as the slightly older, more worldly friend who managed to look fashionable even in the school’s requisite plaid skirt. For them, Lagarde’s appointment summons a time of life that often shapes personalities and is somehow never left behind. It was that way for Christine Lagarde, too.

* * *

To the five-bedroom, split-level house on Michaels Drive in Bethesda, she brought a habit of European independence. She left behind her family in northern France, where her father, an English college professor, had just died, and she entered a more controlled American world, joining the family of Bill Atkins, an executive with the Touche Ross & Co. consulting firm, and his wife, Marion Guion Atkins, an accountant.

The Atkinses had a son, Michael, in middle school, and two daughters: Laura, also a Holton-Arms senior; and Elizabeth, a Holton-Arms sophomore.

“At first, we were like, ‘Christine’s nuts to do one more year of school,’” recalls the now 54-year-old Laura Poisson, a George Mason University clerk, who is married and lives in Rosslyn. “I immediately thought, ‘Wow, she’s a lot more mature than I am, but also, ‘I am going to click with this person.’ ”

In the Atkins home, Lagarde was allowed to smoke only in her bedroom. She missed having a plentiful supply of yogurt. She hated the local tap water so much that she sometimes violated the family’s one-soda-per-day rule. She bristled at the before-midnight curfews.

“I came from the French high school system,” Lagarde says. “I used to be extremely independent when I was in France, but when I arrived in the U.S., it was much about, ‘Who are you going out with? What are you doing?’ I may have resented that.”

Elizabeth Ruhland, the younger Atkins daughter, remembers the adjustment. “A few months into her time, she got comfortable,” says Ruhland, a senior marketing manager at Sirius XM Radio in New York. “One day, she came downstairs for breakfast and said: ‘Guess what? I just had my first dream in English.’ ”

Lagarde and her host father forged a connection that, although neither knew it at the time, would endure deep into her professional life.

“She was older than our children and a little more experienced. She was never, ever a troublemaker, ever, okay?” says Bill Atkins, 72, who now lives with Marion in South Carolina. “She was like a daughter. . . .We would talk with her, but it wasn’t filled with a lot of emotion like some teenagers and children. With her, we’d have really good discussions, whatever was on my mind, something she observed being in Washington.”

* * *

At the beginning of her senior year in 1973, Page 4 of the Scribbler, the Holton-Arms student newspaper, carried the big news: “Holton Welcomes Christine Lallouette.” In a brief interview, the French newcomer expressed surprise that American women were not all “really fat and awful” and instead found them weight-conscious to “an obsession.”

At the end, the article perfunctorily noted her career ambition: “Christine eventually hopes to be a cultural diplomat for France, and plans to study political science when she returns next year.”

She joined the dance troupe and took courses in history, English and math. And even though she ended up with a career involving numbers, she disliked the subject so much that she wrote, “please, no math,” in her yearbook testimonial.

Within the social stratosphere of Holton-Arms, where President Gerald R. Ford’s daughter, Susan Ford, was also a student, Lagarde hung out mostly with a small band of outsiders, among them: Maggie Quiroga Mainor, the Argentine daughter of an international relations executive, and the late Gabrielle “Gabby” Geaslin Swartz, the daughter of a purported CIA operative.

Lagarde herself had an unusual background, which she alluded to in a mostly fictional story that she wrote for the Holton-Arms literary journal Scroll. The piece, titled “Noblesse Oblige,” drew on her mother’s roots in the French nobility during the 1800s.

Still, Lagarde never felt that she or her group quite fit in with her well-heeled and more traditional classmates.

“We were not especially the Holton types. I was a smoker. Gabby was a smoker. We were a little gang. We were not the typical, good, well-behaved, perfect girls,” Lagarde says with a hint of dismissiveness in her voice. “Gabby, she had a car. We’d sneak out and have lunch at Burger King.”

The girls talked boys but delved into politics and philosophy as well, Mainor recalls. Lagarde missed going to a coed school and hated the blind dates. “I didn’t have a boyfriend those days. Not that I didn’t want to. I just think it didn’t work out,” Lagarde says. “Wait — I did have an Australian boyfriend.”

For Mainor, who lives in New York and owns an accounting and human resources firm, the friendship lingers. She remembers how their little group would talk privately in the woods, beyond the edge of the school parking lot. Once, Mainor says, Lagarde helped her cope when a particularly painful dispute erupted between Mainor and her father.

“Christine helped me understand it. One of her traits is to cut to the core of a problem without bashing your parents if the issue was about your parents. Those conversations, I know, she’ll take to her grave.”

In spring 1974, Lagarde graduated from Holton-Arms, wearing the school ceremony’s customary long white dress. Even as she climbed corporate and political hierarchies — becoming a lawyer and chairman of Baker & McKenzie, one of the world’s largest law firms, and then becoming the French finance minister — she never stopped relying on her host family.

“Whenever I’ve had to make a tough decision, I always got back to [Bill Atkins] and asked, ‘What is your view?’ ” she says. “He was a partner at Touche Ross, and he knew how partnerships at firms worked. He was a surrogate father in a way.”

* * *

William Cohen still remembers the intern who read and responded to his constituents’ letters in French. As for Lagarde, she especially remembers Nixon’s foul mouth.

“I was being allowed to listen to the Nixon tapes, and it was like, ‘Holy cow.’ The language was not exactly pure,” she says. “The internship made me think about checks and balances. It was my first injection of political life, and to get that at age 18 leaves a mark on you.”

Cohen hasn’t talked with Lagarde since the internship. Now that he runs a global consultancy in Washington and she’s the IMF head and lives in Georgetown, he wonders whether they might rendezvous.

“Since she’s on the world scene in such a prominent position, I am sure I’ll make every effort to reach back out to her,” he says. “I’ve had outstanding interns in Washington, and I would count her as one of them.”

A very Washington thing to say, of course. And a lesson for those in high places: Always treat your interns well.