The average time spent on academic subjects has declined in American high schools since the late 1960s as enrollments have surged in "personal service and development" courses such as driver education, according to a new report.

Since 1969, the report said, average credits earned by students, which are based on time in class, have dropped by 11 percent in U.S. history, 9 percent in French, 7 percent in algebra and 6 percent in chemistry. By contrast, average credits rose by 75 percent for driver training, 16 percent for coopera- tive education, which provides credit for non-school jobs, and 9 percent for general shop.

The proportion of graduating seniors taking the college preparatory curriculum fell by one-third to 36 percent, the report said, while those in the less demanding general track more than doubled to 42 percent.

"There seems to have been a systematic devaluation of academic courses . . . and a profound shift into the general track," said Clifford Adelman, who wrote the report. Adelman is an associate at the National Institute of Education, the research arm of the U.S. Department of Education.

Adelman called the general track a "vague area of mediocrity," dominated by survey, remedial, and personal service courses. Yet, "more and more of these students are going to college," Adelman said. Even college prep students, he said, are spending less time on academic courses.

Since the late 1970s explanations for widespread decline in high school test scores have ranged from television to divorce to a change in the kinds of students being tested. But Tommy Tomlinson, another National Institute of Education researcher, said the decrease in academic course work may be the most "impressive" explanation.

"There is a finite amount of time set aside for schooling," Tomlinson said. "If more is spent on the soft subjects, then there's less available" for traditional academic work.

Adelman based his study on the transcripts of a nationwide cross section of about 8,800 public high school graduates, collected by researchers at Johns Hopkins and Ohio State universities, between 1969 and 1981.

The study was prepared for the National Commission on Excellence in Education, established by Education Secretary Terrel H. Bell. Adelman said the study is the first comprehensive report in more than a decade on how students are spending their time in high school, although previous studies have shown enrollment declines in specific fields, such as foreign languages and science.

According to the new study, the proportion of high school graduates who had studied U.S. government fell from 88 percent to 51 percent during the 1970s, while the proportion in second-year French dropped from 20 percent to 10 percent and those studying advanced math decreased from 13 percent to 8 percent.

There were substantial rises in the share of students getting credit for psychology--up from 2.4 percent to 24 percent; for remedial English, up from 19 percent to 28 percent, and for training in marriage and adulthood (sex education), up from 1 percent to 17 percent.

Overall, the proportion of school time spent in academic subjects, such as English, social studies, math and foreign languages, fell from about 70 percent to 62 percent, Adelman said. Credits for personal service and development courses, including driver training, chorus and band, consumer education and personal guidance, rose from 8 percent to 13 percent.

Within most academic fields, enrollments fell the most in advanced courses and increased for introductory and remedial work.

The average total amount of courses taken by high school graduates fell slightly, but the number of electives and one-semester courses increased, Adelman said, indicating that the curriculum had become "diffused and fragmented."

Adelman said the changes probably reflect the push in the 1960s for relevance and flexibility, as well as the reduction then in college admission requirements. They do not reflect any increase in the proportion of 18-year-olds graduating from high school, which has remained at about 75 percent since 1964.

Last year scores on the Scholastic Aptitude Test (SAT) for high school seniors rose slightly for the first time in two decades. Adelman said some states have recently started to require more academic courses for high school graduation and college admission.

"I think there is some movement now in a more rigorous direction," Adelman said. "But there are still far too many kids out there with a really bad preparation in high school . . . . You don't want to deny access to college , but access is a fraud unless there is quality."