SCHOOL'S OUT in the District of Columbia, but if you hear some shouting from the rooftops, there's good cause: increasingly, the city's public school students are scoring above national averages on standardized tests. And the younger they are, the better they're doing as they move through the system. Best of all, the progress has been steady for a good five years now-- thanks to a number of simultaneous changes in attitudes, events and personalities that have had direct effects on what happens in the classrooms.

Third-grade students, to take the most impressive group tested, surpassed national norms in math, science, language and reference skills for the first time last year; now they not only have kept improving in these areas, but also have exceeded national norms in reading and social studies. Sixth graders, who last year exceeded national norms only in reference skills, this year scored above national norms in five of these categories, with social studies only slightly behind.

Ninth and 11th graders, whose earliest years in the school system occurred before many of the improvements could affect progress, fell below national norms in all categories. Still, they, too, showed solid gains over last year in every area. So what accounts for this apparent trend toward higher test scores?

The explanations vary, but they are neither contradictory nor mutually exclusive. They start with a definite policy decision, made several years ago, to establish a "competency based curriculum"--a sort of academic checklist of skills to be taught and mastered, grade by grade, course by course. Something else changed for the better at about the same time: the departure of Barbara Sizemore as superintendent permitted more rational discussion of testing itself, and an acceptance of tests as a legitimate though less than perfect way to gauge academic progress.

More order in the classrooms helped as well. Increasingly, principals have been rising to meet their responsibilities to maintain the discipline necessary for teachers to do their work. Helpful, too, has been the election of school board members content to do their jobs with lower profiles, higher levels of efficiency and far fewer symptoms of political insanity. The board's serious majority, led by president David Eaton, also has recognized the creativity and administrative good sense of Superintendent Floretta McKenzie.

The result is an upbeat spirit that surely feeds into the progress. More and more administrators, teachers, parents and students can feel proud about their schools and in turn can sense that their investments in energy are worth it. That is news of the very best kind for the Washington of tomorrow.