Sister Mary McNabb deSales has spent almost three decades as a nun of the Visitation Order here. Rarely - and then only for educational or medical purposes - does she leave the grounds of the convent on 35th Street NW.

But this Sunday, Sister deSales will make one of her rare forays outside the convent to go next door to Georgetown University where, along with two eminent journalists, a Nobel laureate and a distinguished diplomat, she will be awarded an honorary degree - one of the university's highest honors.

The diminutive and self-effacing teacher of mathematics and physis at Georgetown Visitation Preparatory School rises each morning at 5:30 to pray, and she spends most of her waking hours in class, in prayer or preparing for class.

She spends hours counseling her students and listening to their problems because, as she puts it, "at the high school level,they need to be recognized for what they are, and they desperately need to be paid attention to." She leads her students in prayer at the start of each class and she always goes out of her way to pray for any of their friends or relatives who are sick or depressed.

With Grace Hendry, a social studies teacher at Montgomery County's Walt Whitman High School, Sister deSales will be one of two secondary school teachers to receive the honorary doctor of humane letters degree from Georgetown this year.

The Rev. Timothy Healy, Georgetown president, describes the gesture - a first for Georgetown and unusual for any major university in the United States - as an effort to "pay what little tribute we can to the thousands of high school teachers who are represented by the graduating class at Georgetown.

"The educational work that we're trying to accomplish at Georgetown is built solidly and squarely on what happens in high school. In honoring all those many people who helped to ready students for a serious undergraduate curriculum. We are honoring those who represent the continuity of education underneath all our artificial divisions."

Healey said it is his intention that the awarding of honorary degrees to high school teachers will become an annual tradition at Georgetown and that in future years the teachers will be selected from high schools that reflect the "national composition " of Georgetown's student body."

Visitation, a school for girls, and Walt Whitman, a Montgomery County public school, were selected this year because both are "distinguished local schools that send us a lot of their graduates," Healey said.

The first move was to poll the approximately 70 Georgetown undergraduates who were alumni of Whitman or Visitation for names of a teacher who "significantly helped you in preparing for college and who you would like to see at Georgetown honor."

Sister deSales and Hendry won hands down.

"Sister deSales is very concerned about students, and she's always ready to give up her time for them," said Mary O'Donnell, a junior at Georgetown and a 1976 graduate of Visitation. "She makes the courses she teaches really fun. She teaches physis, computer math and calculus and she's the best. I think high school teachers are very important. I still go back and talk to her a lot."

Sister deSales, who grew up in Grosse Isle, Mich, came to Visitation as a boarding student in 1947 "because my parents were afraid I wasn't getting a good education and they were afraid I wasn't getting a Catholic education."

After graduating in 1949, she left to go to college in St. Lousis for a year, but she returned in 1950 intending to join the order after six months as a postulant and a year as a novice.

"I was very impressed with that community of nuns," she said. "They were all very different, but they were wonderful people and I wanted to be part of them."

In those days, nuns were not allowed off the convent grounds, so when Sister deSales ended her novitiate and was ready to resume college level academic work, faculty members from Georgetown came to the convent to tutor her on an individual basis. Because nuns then were not permitted to be alone in the presence of a man, another nun had to sit on the tutorial sessions as a chaperone.

Rules were relaxed a bit in 1955, and Sister deSales was allowed to enroll at Trinity College, where she earned a degree in mathematics with a minor in physis two years later. Subsequently, she earned her masters degree from Catholic University, also in math and physics.

As she runs through a series of complex equations at the blackboard, she often stops in midstream with the observation that "some of you don't understand what the problem is, do you?" Carefully and deliberately, she will repeat the process so that everyone can understand.

She tries to keep her subject matter interesting and in a recent class on computer mathematics she had her students write a program for a computer to play Nim, a game in which players compete by picking up one, two or three pebbles. The player who picks up the last pebble wins.

Every possible move in the game was analyzed as either a winning move with the information to be fed into the computer.

"A winning move is any move that forces an opponent to make a losing move," she says. "A losing move is a move that allows an opponent the choice of making a winning move or a losing move."

In addition to physics and math, Sister deSales has also tried her hand at teaching auto mechanics and building radio sets. During the summers, when school is out, she tends the convent vegetable garden and plays an accasional game of tennis.

But she considers teaching her true vocation, and she interprets the term broadly to include more than just the subject matter.

"School is the only element of stability in their lives for so many of these students," she said. "It's great to be around young people. They keep you young." CAPTION: Picture, Sister Mary McNabb deSales on the Visitation campus. By Craig Herndon - The Washington Post