THREE YEARS ago, Prince George's County School Superintendent John A. Murphy promised that his students would reach the 75th percentile on standardized tests by 1990. When pupils reach that level, they have scored higher than 75 percent of the students tested nationally. That's an important threshold, within reach of highly regarded schools such as those in Montgomery County, where students have hit the 80th percentile. Black students in Prince George's, as a group, had test scores far below those of whites. The superintendent promised to close that gap. Few believed that both things could be done at the same time. They were wrong.

The California Achievement Test (CAT) measures the reading, math, spelling and language skills of third-, fifth- and eighth-grade students. Recently released scores show that Prince George's third-grade students hit the 73rd percentile on the total battery of tests, the highest score ever for Prince George's and an increase of nine points since 1986. Both fifth- and eighth- grade students improved by nine points, up to the 69th and 67th percentiles respectively.

Some black students in Prince George's had already recorded high scores, but more and more have excelled on the CAT. In 1987 black students ranked above the 50th percentile for the first time. This year, third- and fifth-grade black students surpassed the 60th percentile, and the gap in test scores is also shrinking. Black third-grade students, for example, trailed whites by 25 points in 1984, but by only 17 points this past year.

Higher budgets, more teachers and a back-to-basics curriculum have helped produce the overall improvement. Schools such as Dodge Park Elementary have shown that black students (96.4 percent of enrollment) can score as high as any others.

At Dodge Park, teachers expect more from their students. More homework is assigned, and regular exams test skills needed for the CAT. A computer lab has helped math students, and parents are given booklets to test their children over the summer. In 1986 Dodge Park's third graders scored at the 77th percentile. They reached the 94th percentile on the last CAT. That type of performance has helped lift the Prince George's schools to the threshold of excellence.