Prince George's County School Superintendent John A. Murphy yesterday revealed for the first time that black children in the county scored more than 20 points behind their classmates in standardized tests and announced a program to battle that "extremely serious" disparity.

Black students also receive lower grades and participate less often in extracurricular activities, according to a study Murphy ordered a year ago and released yesterday. The superintendent, who has been in his job one year, said he would implement programs to raise the performance of black students that include academic remediation, a pilot program of frequent testing and strict attendance regulations.

"This is delicate data," Murphy said of the racial breakdown of test scores. "It's something we've not looked at before , and that's why it hasn't gotten the attention it needed.

"Now we're going to say there is a serious problem and it's got to be addressed," he said in an interview.

The lower achievement of the county's black students, who make up 57 percent of the 105,000 students in the system, is a problem that has long been recognized unofficially by school administrators, but this is the first time it has been documented. Murphy has for some time spoken of the racial disparity, and since he was hired he has stated his intention to raise the overall county test scores from their current level, at the national average, to at least the 75th percentile.

His focus on the issue accompanies other major changes in the system, including introduction of a magnet school program and a sweeping administrative reorganization.

The racial disparity in the county is typical of school systems across the country. Although the performance of black students nationally on standardized tests rose during the 1970s, it remained lower than that of non-blacks in 1980, according to a study released this year by The College Board, a national organization that administers college entrance examinations.

The county's study showed that Asian students scored highest on the standardized tests last year, followed by whites, American Indians, Hispanics and blacks.

Murphy said he did not believe the difference in achievement reflects less ability among black students. "I think it's expectations," he said, referring to studies that show students perform better when more is expected of them by teachers.

He said teachers often expect less of minority students, a problem that can be corrected through awareness. "It's not intentional," he said.

In a speech prepared for delivery last night at the opening of a four-day workshop for principals, he urged principals to "start setting the atmosphere of high expectations for students."

He said socioeconomic factors, which correlate with achievement, also contribute to the racial disparity because more black students come from low-income homes than white students.

Murphy acknowledged that he expected some opposition from the community to his decision to focus on racial differences in academic performance.

Former school board member Bonnie F. Johns said such racial breakdowns could be helpful if used for diagnostic purposes. "If, on the other side, it's used, as I know it's going to be used, as proof of the inadequacies of black children, then it hurts," said Johns, who serves on an advisory committee for the county's magnet school plan.

"I think it's good the school system is owning up to the problem," said state Del. Albert R. Wynn (D-Prince George's), a member of the Ad Hoc Committee on Quality Education, a predominantly black group that formed over the county's long-standing desegregation problems. "I just hope . . . it is things that are workable, not theory."

The county's study on racial differences found that black students as a group scored lower than white students on the California Achievement Tests in each of the three grades where it is administered, and in all subject areas.

There was also a disproportionate number of black students who during the past school year failed to earn a C average, the criterion required for participation in extra-curricular activities. The study found that significantly fewer black than white students received As in selected high school courses, while many more received failing grades.