THOSE WHO'VE WATCHED and applauded the striking improvements in the management of the D.C. school system and the shared sense of purpose among top school officials during the past two years must feel, as we do, a bit saddened by the decisions of school-board president Therman Evans, and borad members Julius Hobson Jr. and William Treanor not to seek reelection. Their leave-taking is a loss to his community. For each of these four-year members have made significant contributions to cleaning up the debris of the last decade of turmoil in the school system and to moving it in the right direction.

Mr. Treanor, the Ward 2 member, has been chairman of the board's rules, policy and legislation committee two of the last three years. With his guidance, the board has made substantial progress in the prosaic but vital task of codifying and revising board regulations, which previously were in great disarry. Mr. Hobson, the Ward 8 member, has been chairman of the board's finance committee the last three years. His mastery of the intricacies of school finance helped provide the board with a sound understanding of the school system's fiscal status and made him an able spokesman for greater fiscal support of the schools.

As for Mr. Evans, an at-large member, we don't think it's an exaggeration to say that he's been one of history. His self-effacing leadership and willingness to act as the mediator of conflict made him an effective, low-key steward - just what the school system needed as it strove to rebuild itself after a decade of debilitating controversy. Of course, Mr. Evans couldn't have been successful without the cooperation of his colleagues. But his leadership has been largely responsible for the quiet and constructive manner in which the school board has been going about its business.

All three men have also been strong supporters of Superintendent Vincent Reed. At times they have differed with Mr. Reed over specific issues, but they, and other school-board members as well, never overstepped the bounds of professionalism and respect that were so often trespassed by board members and superintendents in preceding years. As Mr. Reed said, their actions underscored their commitment to the schoolchildren of this city. That's an impressive legacy.

The reasons the men are leaving their board seats are varied. But we think Mr. Hobson's reason, particularly, should not go unnoticed. Mr. Hobson said his decision results primarily from the low level of compensation the unsalaried school-board members receive. "The $4,000 we get a year [to cover expenses] just doesn't cover it. I simply can't afford to continue," he said. We think he has a point. The current ceiling on expenses for school-board members has long struck us as much too low. The school board should be made up of a broad cross-section of the community, specifically including those of limited means. Mr. Hobson's leaving may not be evidence of a trend, but we note here that several board members with families to support have left the board in recent years for just the reason given by Mr. Hobson. A city-council committee currently is considering two measures that would increase the compensation for school-board members. Regardless of which measure they finally support, we think the compensation rate should be increased.