WASHINGTON (AP) _ Despite some improvement in the scores and number of minority students taking the ACT college entrance exam, the overall performance of the nation's high school seniors remained unchanged again this year, test officials said Wednesday.

The 1992 average score on the American College Testing assessment, the predominant college-entrance exam in 28 states, was 20.6 - the same it has been since 1989. The 832,217 graduating high school students were given tests in English, math, reading and science, with each scored on a 1-36 scale.

A positive note was that 17.7 percent of this year's seniors who took the ACT identified themselves as minorities, compared to 14.5 percent of the 1988 seniors.

As for the national average, ''We would see holding steady as not all that bad,'' said Patricia Farrant, spokeswoman for the educational services organization based in Iowa City, Iowa. ''ACT scores remain stable as the population tested continues to change.''

Asian Americans scored an average of 21.6 and whites 21.3, both above the national average. Puerto Rican students scored 19.3, followed by 18.4 for Mexican Americans, 18.1 for American Indians and 17 for blacks. Over a five- year period, the scores of minority students increased slightly, from .1 to .5 points, for all groups.

''The stable and in some cases strengthening performance by minority students ... can be explained at least in part by the accompanying increase in the proportions of minority students taking demanding programs of core coursework in high school,'' said ACT President Richard Ferguson.

Ferguson said a typical college preparatory program includes four or more years of English and three years or more each of mathematics, social studies and natural sciences. He said the difference in average scores for students who took the more rigorous courses and those who did not ranges from 2 to 3.1 points.

Fifty-three percent of the graduating seniors taking the ACT had completed a college preparatory course this year. In 1988, about 43 percent of the students had done so.

This year, for the first time, more than half the Puerto Rican seniors took more demanding courses. About two-thirds of Asian American students took the rigorous courses.

About two weeks ago, the College Board reported a one-point increase in the verbal section of the Scholastic Aptitude Test and a two-point rise in the mathematical section for 1992. The total 899 SAT score was on an overall top score of 1,600.

Both the SAT and ACT were developed to test a student's readiness for college. But the ACT is considered a broader test of high school mastery, while the SAT tests are more narrowly focused on math, reading and language skills.