Nearly every school day, headmistress Barbara Landis Chase leaves her office to walk briskly around the rolling hills of Bryn Mawr School. On this cloudy afternoon, she commiserates with fourth-graders frustrated by the complexities of computers, cheers on the lacrosse team goalie and pokes her head into a drama class.

There, a student stands before her peers, her nose high in the air and her fist clenched against her hip. "I'm your worst nightmare," she says like some Cruella De Vil in sneakers, "your new headmistress." Terrible things -- more homework, less socializing, longer uniforms -- are all possible under her haughty regime.

Transfixed by the student's acerbic view of the future, Mrs. Chase laughs, her voice echoing through the auditorium.

The improvisation has a point. At the end of this school year, she is leaving these 26 acres in North Baltimore for a new life some 400 miles away. In July, she'll become the first female head of Phillips Academy in Andover, Mass., an elite boarding school that counts George Bush, Jack Lemmon and Dr. Benjamin Spock among its graduates.

But more than mere miles separate the two. While Mrs. Chase believes the educational philosophies are similar, Bryn Mawr and Andover are a study in contrasts: all-girls vs. coed; 758 students vs. 1,200; a $9,610 tuition vs. $18,500.

And although Andover is considered a liberal school where students shun uniforms and buy condoms at the infirmary, it routinely dismisses as many as eight students a year for academic and social offenses. In 14 years at Bryn Mawr, Mrs. Chase has dismissed only two, events she still counts among her most wrenching decisions.

But like nearly everything in her life, she meets the unknown with a controlled, thoughtful demeanor that seems both inherited from her Mennonite ancestors and learned from her experience in management.

Sitting in her office eating a tuna fish sandwich and chocolate cookies, she watches the lower school students race by her office -- skipping and giggling and looking blissfully impervious to life's disappointments.

"There's something about a leave-taking that feels like abandonment, even though rationally you know it's not," says Mrs. Chase, who informed the school of her decision during a class assembly in February. "I wanted them to know I hadn't been dissatisfied, that I would miss them. . . . When you're in a school, you're naturally inclined to think about the lesson in everything. The lesson here was that change is absolutely necessary for all of us to grow and move forward in life."

With her chestnut brown hair, trim build and classic taste in clothes, she's hardly the stereotypical headmistress in sensible shoes and a silvery bun. She has a penchant for wildly colorful watches. And at age 49, she still retains an almost schoolgirlish smile, even with her bifocals on.

Her conference table is stacked with letters attesting to her popularity. One corner holds gifts: a porcelain pig (she collects them), a book from alumnae, a photo of Bryn Mawr students, faculty and administrators.

On a larger scale is the $50,000 donated to the scholarship fund in her name, and the dance studio and theater that will be renamed the Barbara Landis Chase Performing Arts Center. When she received the last recognition at a trustees' cocktail party recently, she said modestly: "A building named after one? One should at least be dead."

All this serves as testament to how one person -- with the help of others -- can transform a place like Bryn Mawr with its pre-kindergarten through 12th grade classes. From diversifying the student body and bolstering faculty salaries to dressing up for Halloween and running with the cross-country team, Mrs. Chase has managed to walk the line between serious headmistress and class clown.

"What other job can you have where you get to raise money, balance a $10 million budget, hire people and . . . at the same time drive the tractor in the spirit parade?" she asks.

But if the job has taught her anything, it's how to hold her emotions in check during critical times.

A little switch

"A long time ago, I learned to flip a little switch that keeps me talking, when if I allowed myself to feel what I was really feeling I would dissolve," she says.

The switch has been in danger of short-circuiting lately.

And it will be put to the ultimate test on June 10, when she delivers the graduation speech, an honor the seniors bestowed upon her this year. As she stands in the daisy-filled graduation garden, her ruminations about beginnings and endings are likely to stir many.

At opening convocation in the fall, she prepared the seniors for the tumultuous year ahead with these words: "This will be the first of many lasts in your lives."

What Barbara Landis Chase didn't realize then was the person she could have been talking to was herself.

The key to understanding how Mrs. Chase fell in love with learning is locked away in Lancaster County, Pa., deep in the heart of Amish farmland, where she grew up in a modest stone house, one of four children born to Dr. Floyd and Ruth Landis, a family physician and nurse.

The one-room influence

In public school, she excelled without much effort. A tomboy who played tennis and field hockey, she reveled in riding her bike through the green hills, past wide wooden barns, windmills and silos.

What made the greatest impression, though, were the one-room schoolhouses she passed and the Amish children studying there.

"It was so picturesque," she says now. "I would be out there just watching them. Part of the fascination was that I could never live it, although I had this romantic notion that I'd come back to teach in a one-room schoolhouse. . . . In a sense, being at Bryn Mawr was kind of an extension of that."

She entered Brown University in 1963, studying history and declining her parents' advice to take teaching courses. College was a heady, exhilarating experience, and one where she also found love. She and David Chase, a student of architectural history, married three days after graduation.

They moved to Providence, R.I., where they had two children, Ashley, 25, now an editorial assistant at HarperCollins, and Katie, 19, a freshman at Brown.

After Katie started school, Mrs. Chase taught music at a private day school, before moving to Wheeler School, another private institution. She taught music, history and English and eventually became the director of admissions.

In 1980, the head of Wheeler nominated her for the position of headmistress at Bryn Mawr. She got the job at age 35, becoming one of the youngest women to run the school in its 109-year history.

"They took a real chance on me," recalls Mrs. Chase. "At the time, one trustee said to me it was a real leap of faith, and I said, 'You're right, it is.' "

Teachers recall how she spoke nervously at those early meetings, fumbling with her watch and stumbling over her words.

Biggest crisis

Her biggest crisis, she says, came a year after she took over. A sixth-grade student died suddenly of a cerebral hemorrhage. The school was overcome with grief, and Mrs. Chase called for a memorial service. On campus, she points out a garden and plaque by a cherry tree near the middle school in honor of the child.

"She was the only student to die while I was here," she says. "It was very powerful, very hard. . . . As the head of the institution, you feel a sense of responsibility for everyone."

Her greatest accomplishments have included improving the racial mix at the once nearly all-white school (25 percent are minorities now), introducing community service to the curriculum and raising money for faculty salaries and the endowment fund.

"She listens well, counts on colleagues but is confident enough to make tough decisions," says Peter Baily, headmaster of Oak Lane Day School in Pennsylvania, who has known her for seven years. "She clearly loves the school and is committed to women's education."

When David M. Underwood, Andover's president of the board of trustees, began researching Mrs. Chase's background, he was surprised at what he discovered.

"We got absolutely no negative references," he says. "That caused us to go to another level of checking. We went to other moles we had in secondary education, and we still couldn't find anything. To me, that means that when she makes a mistake she acknowledges it, and people don't hold it against her."

With students, Mrs. Chase wins praise for being down-to-earth. They rave about her rendition of "How the Grinch Stole Christmas"; her faculty-show parody of "A Teen-ager in Love," which she sang in an old motorcycle jacket; her participation in the celebrity basketball game against the Orioles.

'One of us'

"She can be one of us," says Joelle Novey, 15, who has been here since kindergarten. "Knowing that she's the big cheese always makes you feel better."

Says Mrs. Chase, "Some people worry about seeming appropriately powerful or august enough. So much of that just screams out when you say 'headmistress' that you really have to work against it. . . . Generally when we're depicted in literature, we're a fairly foolish bunch. And the images of women are almost not there. It's all Mr. Chips."

If she were depicted in literature, what image would she choose?

"I always think of myself as a teacher . . . in the largest sense of that word," she says. "And the best teachers are always learning."

There will be plenty to learn at Andover, a sprawling 600-acre campus for ninth- through 12th-graders that serves as a training ground for the Ivy Leagues.

A lot to learn

With 140 buildings, the campus itself resembles a college. On a clear spring afternoon, students sunbathe on the grass, while their more enterprising classmates rehearse "Antigone" on the steps of Phillips Hall. The smell of chocolate chip cookies wafts from Commons, where students share meals in the wood-paneled dining rooms.

"It's like a village in the sense that you know and are known," says Vic Henningsen, a history instructor and Andover graduate (class of '69). "But it can turn into Salem Village real quick."

With rival school Phillips Exeter Academy in New Hampshire having appointed a woman head recently, several students felt that it was time for Andover, which became coed when it merged with nearby Abbot Academy in 1973, to do the same.

"Some people think she's the token female head, that she's there to make us seem liberal and politically correct," says Abbie Suberman, 18, a senior from Chapel Hill, N.C.

While the board calls that "baloney," Mrs. Chase, who downplays her role as the first woman, acknowledges that she will have to work harder to win over some factions.

Male students may be one. Some have voiced concerns about her limited background with young men.

"I was taken aback," says Joe McCannon, 17, incoming school president. "Just because she's been at an all-girls' school doesn't mean she's not capable of running a school with boys. But it was like, 'Geez, I'm a boy. Is she going to know how to deal with me and the other boys?' "

There will be plenty of other issues to confront: The six-day week, in which students routinely have Saturday classes, is controversial; some students have recommended that lesbian and gay faculty, who live on campus, be allowed to room together; and recent seminars about drug addiction and condom use show how boarding schools are confronting social ills.

"Part of the teen-age mentality is to press limits," says Ms. Suberman, who has seen six friends dismissed from school for drinking or drug violations. "We definitely press them here."

All this awaits a woman who wasn't even looking for a new job.

The call came in mid-October, and after consulting her family, Mrs. Chase formally applied. By winter, the 200 applicants had been whittled down to three: Mrs. Chase and two men.

Her resume had its limitations: She had no experience with boarding schools; she had been in a girls' school for years; and she had no Ph.D (although she does have a master's from the Johns Hopkins University).

No chinks

But if she had drawbacks on paper, she made up for them in person.

"Every time we saw her, she got better," says Mr. Henningsen. "When we saw the others, we began to see chinks in their armor. We heard the same joke once too often. In the end, that spelled Barbara Chase."

In July, she'll move from her white cottage at the base of Bryn Mawr to a Federal-style house with formal gardens on Andover's campus. For the first year, she will live there with the family cat, Max. Her husband will remain behind, finishing his work as executive director of Preservation Maryland, a private statewide preservation group.

As much as the job itself, this personal upheaval concerns her. "We're going to be using USAir a lot," she says with a laugh.

With her days winding down, she's writing an individual letter to each senior, a tradition she started several years ago. And she's struggling with her graduation speech. What is it, she wonders, that she wants them to know?

"Whatever I did -- whether I did it well or badly -- I was doing for the good of the kids," she says. "That was always my bottom line."