American education has deteriorated so drastically in the past two decades that "our very future as a nation and a people" is threatened, an education commission warned yesterday in a study released by the White House.

To halt a "rising tide of mediocrity," the National Commission on Educational Excellence recommended a wide range of educational overhauls, including seven-hour school days, 200- to 220-day school years, "far more homework," more required courses for high school students, higher teacher pay, standardized achievement tests for entrance to college and higher standards at all levels from first grade through the university.

"If an unfriendly foreign power had attempted to impose upon America the mediocre education performance that exists today, we might well have viewed it as an act of war," the study said. "We have, in effect, been committing an act of unthinking, unilateral educational disarmament."

The panel, appointed by Education Secretary Terrel H. Bell, called on the federal government to provide "national leadership" and to meet a variety of other responsibilities toward education. But it did not criticize Reagan administration moves to cut federal education aid and dismantle the Department of Education.

President Reagan received the report at a White House ceremony and told commission members and 120 other education and civic leaders that he would "listen closely" to its recommendations.

Reagan said the findings show that "our educational system is in the grip of a crisis caused by low standards, lack of purpose, ineffective use of resources and a failure to challenge students . . . to the boundaries of individual ability."

"Our agenda is to restore quality to education by increasing competition and by strengthening parental choice as local control," he said. "We'll continue to work . . . for passage of tuition tax credits, vouchers, educational savings accounts, voluntary school prayer and abolishing the Department of Education."

Bell, at an earlier briefing, strongly endorsed the report and called it a "blueprint for educational renewal."

National education groups generally praised the report's call for higher standards, also the main thrust of state and local school change efforts in the past few years. But most groups criticized some of the panel's specific recommendations and said the report strengthened their own long-standing requests for greater education spending.

"There is simply no inexpensive shortcut to educational quality, no magical panacea," said Willard McGuire, president of country's largest teachers' union, the National Education Association. McGuire said the report "reaffirmed what teachers and the NEA have been saying all along, that education in our country needs a shot in the arm."

The report, entitled "A Nation at Risk: The Imperative for Educational Reform," was approved unanimously by the 18-member commission headed by David P. Gardner, president of the University of Utah and president-designate of the University of California system.

Other members included A. Bartlett Giamatti, the president of Yale University; Glenn T. Seaborg, a Nobel Prize winner who teaches chemistry at the University of California; former Minnesota governor Albert H. Quie; Margaret Marston, of Arlington, Va., a member of the Virginia State Board of Education; and Jay Sommers, foreign language teacher from New Rochelle, N.Y., last year's National Teacher of the Year.

The group said signs of educational deterioration in the post-Sputnik era extend far beyond a major decline in average scores on the Scholastic Aptitude Test (SAT) for college admissions, which turned up slightly last year for the first time in almost two decades.

It said remedial, high-school level mathematics courses had increased by 72 percent between 1975 and 1980 in public four-year colleges, and now make up one-quarter of all math taught in those schools.

About 13 percent of all 17-year-olds can be considered "functionally illiterate, it said, by simple tests of everyday reading, writing and comprehension.

Its lengthy catalogue of recommendations--all addressed to school and college officials and state legislatures--include:

* Every student seeking a high school diploma should be required to take four years of English, three years each of mathematics, science and social studies, and a half-year course in computer science. Two years study of a foreign language are "strongly recommended" for those going to college.

* Schools should "strongly consider seven-hour school days, as well as a 200- to 220-day school year" to provide significantly more time for learning these "new basics." Most American students now take five to five and a half hours of classes a day for 180 days a year.

* Promotion and graduation policies should be based on "academic progress . . . rather than rigid adherence to age." But it said standards should be far higher than the minimum competency tests now required by many states, which it said have tended "to become the maximum, thus lowering educational standards for all."

* Colleges should adopt "more rigorous and measurable" admissions standards, not only by requiring applicants to take more academic courses in high school, but by demanding that they reach specific "levels of achievement on standardized achievement tests" in the five basic subjects, and in some cases in a foreign language.

Gardner said exams should be taken in addition to the SATs that are now widely required, and that each institution should set and publish widely its own qualifying scores, depending on the quality of students it wishes to enroll.

* Teachers should get higher pay and 11-month contracts, instead of the 10-month contracts they generally have now.

But the report said pay should be "market-sensitive and performance-based," reflecting demand in particular fields and evaluations that are done partly by other teachers.

As in universities, it said, better teachers should get higher ranks and higher pay, an idea that has long drawn sharp opposition from teacher unions.

Although state and local officials have the "primary responsibility for financing and governing" schools, the report said the federal government "has the primary responsibility to identify the national interest in education" and should "help fund and support efforts to protect and promote that interest." It said the federal government also "should help meet the needs of key groups of students" such as the gifted, the handicapped, "minority and language minority students," and "the socioeconomically disadvantaged."

In effect, this endorses the main federal aid programs, most of which the administration has sought unsuccessfully to replace with block grants. The report follows 20 months of hearings and debate. The panel also commissioned papers by 40 education experts.

While the report said "excellence costs," the commission put no price tag on its recommendations. "A good deal can be done within existing resources," said Gardner, adding that when some changes are carried out, "we can ask for more money, and then I think we will get it."

Gardner said one of the study's most important findings, published several weeks ago, was that high school students spend much less time on academic subjects than they did in the late 1960s, and much more time on "personal service and development courses," such as driver education.

"In effect, we have a cafeteria-style curriculum in which the appetizers and desserts can be easily mistaken for the main courses," the commission report said. The proportion of students in the college preparatory curriculum has fallen, while those in the less demanding general track have more than tripled.

Overall, the commission said, even though the "average citizen . . . is better educated" than a generation ago because more people stay in school longer, the average graduate of high school and college "is not as well educated."

"The ideal of academic excellence as the primary goal of schooling seems to be fading across the board in American education," the report said. Instead, it said, there is "shoddiness" and a "national sense of frustration" as "more and more young people emerge from high school ready neither for college nor for work."

Gardner said the panel's relatively short report, 36 pages, is "itself a reflection of the times."

"The attention span of most people tends to be relatively abbreviated," he said.