The Madeira School, 366 rolling, resplendent acres, roughly one acre per student, celebrated its 75th anniversary of decorous, tradition-filled existence yesterday afternoon on the campus in Fairfax County.

As the grounds filled with alumnae and guests, there were reminiscences of teachers past, teas beneath the dogwoods, Christmas pageants, the outing club and skids on floors made slick by soap and powder.

Praise was heaped upon all the Madeira headmistresses. Except one. Jean Harris. She served three years at Madeira. Then she killed Herman Tarnower, the diet doctor. She's serving a 15-year prison sentence in New York, in a cell she has described as a "cozy room." She still teaches, but it's inmates now.

At Madeira, they'd like to forget about that. The 75th anniversary flyer treats her with the utmost economy. "1977 . . . Mrs. Harris arrived as sixth Headmistress. . . . 1980 Mrs. Harris resigned."

"Nobody talks about it," said Adrienne Ebert, who stood with her fellow sophomores on a terrace overlooking the Potomac and a valley of brilliant, haze-draped trees. "Nobody associates it with the school. That was her business and this," said Ebert, "is our business."

The alumnae feel the same way.

"It's all behind us, really," said Eleanor Peirce Koeghler, class of '33.

"It's sad," agreed Kay Pearson Porter, who graduated four years earlier, when Madeira was still downtown at Dupont Circle.

"Yes," said Madeira board member Kathleen Bell. "We have more applications now than we've ever had. The whole thing," said Bell, "was impeccably handled."

"Yes," said Allegra Maynard, who was the school's second headmistress, and took over after Lucy Madeira, the school's founder, retired in 1957. "I'm so happy with the new arrangement."

The "new arrangement" is Charles McKinley Saltzman II, the new headmaster, the first male principal in the school's history. He grew up in Cleveland Park, went to St. Albans and Harvard, wears impeccable suits, keeps his trousers preppie-short and looked thoroughly at ease with himself during the afternoon ceremonies.

The ceremonies took less than two hours, time normally allotted to student-teacher meetings, so the girls lost no school time. They, in their schoolday best, marched in file down to the center stage of the school's chapel/auditorium, where they listened respectfully to their headmaster's remarks (about the importance of "educating the heart" as well as the mind) and those of Smith College president Jill Ker Conway.

"It's not fashionable these days to encourage young people to look for a calling rather than careers with material rewards," Conway told the students, and then advised them to think about balancing the two.

By the end of her speech, it was time for music again. Then benediction, then the school hymn. The name Harris had not been uttered, but that didn't mean she had been forgotten.

"A lot of the new girls here have no conception of it," said Rachel Pettus, a senior from England. "She was really wonderful. She helped my roommate do a lot with the advanced Latin program. After that, my roommate used to write to her."

"I didn't know her that well," said Cathy Stewart, a senior. "She wasn't one you'd ever be intimidated by. She was so frail."

"It's not something you forget," said Paige Collier, another senior. "I remember when we were sophomores she invited us in from the cold for hot chocolate and biscuits the first month of school."

"I just really miss her," sighed Cricket Stone.