One of the nation's largest and most prestigious independent day schools, The Sidwell Friends School in Washington and Bethesda, starts its centennial weekend celebration today with a flourish of fireworks, square dancing, and 17 class reunions.

One hundred dogwood, pine and cherry trees have been planted on the 15-acre Sidwell upper school campus on Wisconsin Avenue near Tenley Circle. (Grades kindergarten through fourth are located in Bethesda.) Three bands will play everything from jazz to bluegrass. Photographs and artifacts of memorable times, like the days when President Theodore Roosevelt's son went to Sidwell and the school's football team practiced on the White House lawn, line the school's arts center.

"There'll be a model of the school's founder, Baltimore-born Quaker educator Thomas W. Sidwell in a carriage, a history of the campus as models of old buildings go up in smoke and then the new buildings," said Nancy Harter, Sidwell's centennial observance coordinator, as she waded through a jungle of boxes from the school's archives long enough to gleefully describe tonight's fireworks display.

"It's a big thing. They have really put a lot of work into this," said Megan Decker, a 17-year-old 11th grader at Sidwell. "In a way, I think it's a little overwhelming."

But that's hardly the consensus at Sidwell, and decision by consensus in the Quaker tradition is the way the school operates, no matter how long it takes. When the school's trustees meet to work out the annual budget, for example, the sessions begin and end with periods of silence. No vote is ever taken. Like a courtroom jury, they work at it until every trustee is satisfied.

On the eve of the 1,000-student, coeducational school's 100th anniversary, Sidwell's methods are apparently working. The school's financial status is the best it has been in decades. Its endowment has increased from $200,000 five years ago to $1 million. The $4 million centennial fund drive has resulted in the renovation of science facilities, a new gym, an arts center, and a Quaker meeting room where groups of students and teachers meet once a week for a period of meditative silence.

"There's a sense of joy and achievement. We have been preparing for the centennial for the last five years," said Sidwell headmaster Earl G. Harrison Jr. "The centennial years have served as a threshold to rekindle alumni interest and a sense of renewed mission."

Harrison said that Quakerism emphasizes tolerance, the peaceful resolution of conflicts, and "the nurturing of the individual in a communal setting. There is utter spiritual equality between teachers and students."

Women have also traditionally held an equal status at the school, said Harrison. The president of the board of trustees is American University professor Susan Webb Hammond, and Saturday's centennial address will be delivered by Sidwell graduate and University of Chicago president Hanna Holborn Gray. Roughly half of the student body is female, 15 percent minorities.

Although 40 percent of its students are the children of regular federal employes, the sons and daughters of the diverse likes of Richard M. Nixon, Robert F. Kennedy and Dan Rather have been educated there.

It's a relaxed atmosphere where smug dogs owned by the faculty saunter around the grounds and lounge in classrooms. When Alice Dater's sixth grade history class gets a little noisy, other students tell them to be quiet, not the teacher.