THE ACHIEVEMENT test scores that the Prince George's County schools published this week are additional evidence of a profoundly important kind of progress. They were not only the highest scores since the county began using these tests. They also show a diminishing gap in achievement between black children and white, while the white children's scores continued to rise.

The test is given annually to third, fifth and eighth graders. The third graders' average score was at the 64th percentile, meaning that it was higher than the scores of 64 percent of the children who took the test throughout the country. The fifth graders were at the 60th percentile, and the eighth graders at the 58th.

Perhaps that should not seem unusual. Most of Prince George's is solidly middle class, after all. But the Prince George's system is one of the largest in the country, and three-fifths of its children are black. The dimensions of the improvement reflected in these scores is a matter of intense national interest as well as local pride. The average black third and fifth graders in Prince George's have now reached a level of achievement higher than the average for children of all races nationwide.

In a pattern that is familiar wherever a school system tightens standards, the most dramatic gains are in the primary grades. It is much easier to keep children from falling behind than to help them catch up. That's why the scores are higher among the third graders than among the fifth graders, and higher among the fifth graders than among the eighth graders. But in the high schools, things are not going badly.

The state now requires all high school students to pass a standard reading test before they graduate. In Prince George's this year, 90.5 percent of the ninth graders passed it. The proportion was down by a hair's breadth from last year, a disappointment. And the test demands only a minimum acceptable ability. But it is set at the ninth-grade norm and, if nine out of 10 ninth graders can pass it, that's a pretty respectable rate.

Credit for the rising scores is widely shared. Superintendent John A. Murphy has imposed strict performance standards on the system, school by school, and that's part of the explanation. Principals and teachers are clearly responding well. A new atmosphere seems to be taking effect. Demonstrations of progress encourage the county to provide greater resources, and greater resources encourage further progress. Seeing their scores rise, children begin to take their scores -- and the skills they reflect -- more seriously. There is no endeavor in which expectations more rapidly become self-fulfilling than in education. In the Prince George's schools, the expectation is increasingly one of success.