They did it for a principal.

More than half of the 1,500 students at William McKinley Senior High School in Northeast Washington walked out of classes yesterday morning, some of them chanting and carrying hand-lettered signs, to protest the reassignment of acting principal Mary Brown.

About 125 of the students then marched to the Board of Education's offices downtown for a meeting with public school Superintendent Floretta D. McKenzie -- at which they were gently but firmly told that the decision would stand.

Brown, who has been the acting principal of McKinley since the summer of 1981, applied for the job on a permanent basis. But last week the Board of Education announced it had approved McKenzie's recommendation of Bettye Topps, an assistant principal at Spingarn High School, to begin as principal Oct. 1.

The students said they had nothing against Topps, and indeed they generally knew nothing at all about her. But they were adamant in their support of Brown.

They offered few specific reasons for wanting Brown to remain. She did a good job in keeping outsiders away from the school, they said, and she made sure that their lockers were not broken into and was supportive of student activities. Beyond that, the students said they just liked Brown, and saw no reason for a change.

Kurtland Johnson, a junior at McKinley, wore a sign taped to his chest that read "We Want Brown." Shortly after 10 a.m., when the walkout began, he took a seat with his classmates on the metal bleachers overlooking McKinley's football field. "Our complaint is that Brown was acting principal, and as long as she's been here she's done a good job," said Johnson.

Jacqueline Pitts, a senior, said, "It was wrong to have Mrs. Brown here for almost two years and then snatch her out." Tarita Bumbray, a junior, said, "I don't see why she has to leave. She's done a great job."

Students said the walkout was conceived and organized by word of mouth. Assistant superintendent Gary Freeman, who heard about the possibility of a walkout on Wednesday and went to monitor the situation yesterday, said Brown would be reassigned but her new job has not yet been determined.

After leaving their classes and asssembling on the bleachers, the students listened to Freeman explain that while Brown's was among the final three names sent to McKenzie for consideration, the superintendent selected Topps and the Board of Education approved her recommendation.

Jackie Shephard, a senior, stood up and pointed to the crowd of students around her. "It's clear we have a majority of the students here in support of Mrs. Brown," she said. "Isn't it the responsibility of the schools to give the students and parents what they want?"

Freeman waited for the applause to stop. "We should give Mrs. Topps a chance," he finally said, but he conceded that "maybe next time we should come to talk to the students and the faculty about it; maybe we should have."

Then Freeman smiled. "Usually we don't get student support like this for a principal," he said. "Usually it's the other way around."

Freeman said no disciplinary action would be taken against students who did not return to classes.

Reached later in the day, Brown said of the support, "I just don't have words. I was just astounded. To tell you the truth, I'm a little choked up about it."

Topps, for her part, said she did not think the situation would be an obstacle for her. "I'm sure it will be a challenge, but not a problem," she said.

About 125 students who had walked from McKinley, at Second and T streets NE, to the Board of Education offices at 12th Street and Pennsylvania Avenue NW, marched in a circle, chanting, "School board here we come, McKinley Tech's gonna give you some," and "We walked here all the way, you're gonna hear what we have to say."

Upstairs, in a 12th-floor conference room where McKenzie met with 25 students representing McKinley, the mood was more sober. McKenzie said it was regrettable that students were never officially informed of the administrative changes at McKinley.

"The principalship is not an elected position," McKenzie said. "I would be a very poor superintendent if I told you the selection process was a popularity contest."

Some students got up to say they felt "left out" and "uninformed," but they did not cite any specific reasons why they thought Brown should stay at McKinley.

McKenzie said she hoped that in the future students would voice their complaints through the board's student services committee and through Tanya Turner, the nonvoting student member of the board.

By noon, when the meeting ended and the shouts from the sidewalk began to die out, it was clear to students that next week they would have a new principal. Nevertheless, the relationship between the students and McKenzie seemed warm and conciliatory.

"You know I love you," said McKenzie as she traded hugs, kisses and handshakes with the students.