EVERYBODY KNEW for months last year that it was coming, and when Floretta Dukes McKenzie finally did announce in the summer that she would be leaving her job as D.C. school superintendent the search for a successor was supposed to move along smartly. The idea was to make the transition without an interim administrator and to have several months for the outgoing and incoming superintendents to work together.

But as Mrs. McKenzie gradually withdrew in anticipation of a February departure, the school board's search bogged down. When the initial response from applicants fell short of expectations, the application deadline was extended from Dec. 31 to March 15. Plans now call for selection of a new superintendent in May, to take office by July. Obviously, extreme care should be taken in the appointment of the next superintendent, but the considerable momentum that Mrs. McKenzie generated should not be lost while everyone awaits a changing of the guard.

Andrew Jenkins, who has been Mrs. McKenzie's deputy for the past six years and who is a 27-year veteran of the city's public school system, will serve as acting superintendent. As one of four finalists for the top job in 1981, when Mrs. McKenzie won it, Mr. Jenkins is actively seeking it again and says he won't let the interim period become an excuse for administrative inertia. That's important, but there is also a danger that an acting superintendent can become hyperactive in campaigning for full title to the top job.

Whatever else Mr. Jenkins may have in mind for the school system, he has the immediate task of trying to win D.C. Council approval of the system's proposed budget of $482 million. Mayor Barry's version seeks to cut that figure to $448 million -- which could pull the rug out from under Mrs. McKenzie's progress over the years toward smaller classes and away from dilapidated facilities. If the ultimate allocation comes out closer to the mark of the mayor than of the school board -- and this year the city isn't exactly rolling in revenues -- Mr. Jenkins may wind up in hot political water back at the board.

But this is a key part of the responsibilities that the next superintendent must assume. Immediate past board president R. David Hall, who is heading the search, has said the city should look for someone who not only can run a school system but can also "understand and deal with the political structure." On top of all this is the unfinished business of Mrs. McKenzie: lifting up achievement levels that still aren't high enough, doing something about the pathetic state of athletics in the city's schools, improving career guidance and college placement efforts and coping with a frighteningly violent and widespread drug market.