Once hailed as among the most successful public school systems in the country, with firm tax support and high-scoring graduates, California's high schools have fallen into a serious slump.

The tax-cutting fervor that climaxed with the passage of Proposition 13 in 1978 is being blamed, in part, for a sharp drop in California seniors' test scores and lack of preparation for college when compared with seniors in other states. But there are other factors as well: an increase in the number of students taking "lifestyle" instead of more traditional academic courses; a lessening of requirements for high school graduation, and a tendency of some high school students to take life easy and to denigrate careers and hard work.

"California now finds itself in the uncomfortable position of wearing a national dunce cap," said the state board of education president, Ann Leavenworth. Although many factors seem to have influenced the decline, most attention is being focused on the school-spending limits imposed by Proposition 13, part of a tax-cutting mood that continues to sweep the country. Critics of the school system cite these developments:

* In the past six years, California has slipped from 12th to 49th place, just ahead of Nevada, in the percentage of personal income spent on public schools, according to the National Education Association. Several high schools have shortened their school day and elmininated many of their advanced courses.

* California high school seniors' reading and language scores have dropped in the last decade from about midpoint to the bottom third of a national percentile scale.

* A College Board survey of California students in the top third of their high school classes showed them taking significantly fewer math courses than such students in other states. Equivalent gaps were found in other subjects.

* Stanford University, one of the state's premier private colleges, reported a four-point drop in just one year in the percentage of California applicants meeting its entrance requirements, while the proportion of qualified out-of-state applicants remained steady.

California officials say they see no immediate relief from the budget cuts, particularly since a state surplus that had cushioned the Proposition 13 tax cuts has now been spent. The most important impact of the cuts may be on teachers, they say. The cuts have affected the quality of instruction by increasing class size, limiting special projects, reducing pay raises and leaving the best teachers feeling "battered and tired," said Don Hill, a high school teacher and member of the elementary school board in San Mateo.

Ellis Bowman, special assistant to the state board of education, said school districts, which hold the right to set graduation requirements, no longer demand as much mathematics, history, English or science as they used to, a response to the cut in the school day and several other pressures.

Art, music and sports have borne the brunt of the cuts, said Reno Lorenz, vice principal of Los Angeles' Chatsworth High School, which has cut the school day for juniors and seniors from six to five periods. "But the solid courses are going to be more restricted also. Higher level courses are seldom given and if there is a conflict, the kid loses out," he said.

Student test scores, in a state always at the forefront of educational innovation, have also appeared to suffer from the advent of new, more "relevant" courses introduced in high schools during the late 1960s and early '70s. Seniors interviewed at South Pasadena High School, where most of them take only a half-load because of new rules, said students gravitate toward one of the new courses, "human physiology," as an easy way to meet the science requirement without wrestling with chemistry.

The University of California, with campuses throughout the state, has encouraged this attitude by admitting freshmen strictly on the basis of grade points without noting the difficulty of the courses taken. A Los Angeles parent, Steve Jacoby, said that his son Martin was refused admission to the University of California at Los Angeles because of low grades. Martin is now attending a local community college, and through self-study is reading novels in Chinese at a fourth- or fifth-year level.

Robert Keavney, now enrolled at UCLA, remembers having a heated argument with his father, a high school principal, when he decided to join many of his friends in taking a light load his senior year. His senior year at Arcadia High School consisted of one difficult English course, plus three courses including physical education and office monitor, which demanded little or no work. His father wanted him to take a fourth year of mathematics, but Keavney was concerned about maintaining the 3.6 grade average which insured his admission to the university. "I didn't see any point in taking a hard course and getting a bad grade," he said.

The shock of life at UCLA, however, was considerable, and at age 22 he is still two years from graduation. He said he realized too late "that the grading in high school was unbelievably easy." Looking back now, he said, "it never seemed that there was any homework."

The heavy influx of Hispanic students, who have unusual problems adjusting to English-speaking schools, may have had some impact on California's 10-year test score drop, from the 52nd to 34th percentile in reading and from the 42nd to 28th percentile in language for seniors. But national surveys also show a decline in the rigor and number of courses being taken by California students, even those in the top third of the class who plan to go to college.

According to a survey of more than a million high school students who took last year's Scholastic Aptitude Test, 65 percent of college-bound senior males take at least two years of physical science, but in California it is 50 percent. For females, it is 50 percent nationally but 30 percent in California. The national average for males taking at least four years of mathematics is 67 percent, but in California 57 percent. For females it is 51 percent nationally but 38 percent in California.

Great differences in preparation also appeared in a massive survey of 58,000 high school students released by the National Center for Education Statistics. In percentages of students taking at least three years of mathematics and science, the West (with California students the dominant factor) finished last. For mathematics, the percentages were: Northeast 48 percent, South 36 percent, North Central 29 percent and West 24 percent. For science, it was Northeast 35 percent, South 20 percent, North Central 21 percent and West 15 percent.

In response to a perceived overall lack of preparation, the University of California has added a fourth year of English to its entrance requirements. Some state officials think the concern over the high school slump has helped arrest the decline of Scholastic Aptitude Test scores among California seniors, which are now at about the national level.

The survey also seemed to confirm a suspicion among many California educators that the state's good weather, beaches, mountains and highways also drain school time at a rate not experienced elsewhere. About 35 percent of the West's students report they were late to school at least five days, or absent for reasons other than illness at least five days in the year. In other regions the figure goes no higher than 20 percent.

California has helped lead the nation during the last decade in a significant alteration of the way public high school students spend their senior year. Because of lowered course requirements, new pressures on family budgets and loosened campus rules, four-year high schools in California "have become in a way almost a three-year experience," said Hill, a teacher at San Mateo's Aragon High School. Many seniors fulfill their requirements for graduation at the end of their junior year, and find it easy in their senior years to win permission to leave school by lunch time, particularly if they have jobs.

John Johnson, 17, a transfer student, said that at his old school, Cardinal Hayes in the Bronx, seniors had no chance to leave early, but he likes the new system, also popular now in the Washington area. Of 23 students in a South Pasadena High School civics class, one of the few required courses for seniors, 18 said they were taking less than a full schedule.

"A lot of people see their education as less and less important," said Danielle Maize, 17, who wants to study dance and acting. "I think a lot of people don't go to college because they don't think it's necessary to have a career."

With population growth slowing, getting into college has become easier for this generation. Students at South Pasadena High who are 18 years old do not even need a high school diploma to enter Pasadena City College. Surveys indicate good high school grades are easier to get now. The National Center for Education Statistics reported a 4 percent increase in As and Bs since 1972, and the College Board said California seniors, despite fewer courses, report better grades in them than the national average.