One of the nation's largest industries is poised this week to begin another record-breaking year: spending more money, using more facilities and launching more projects than ever before, and at the same time serving fewer customers than at any time since 1968.

The industry is education, which this year will account for $7.30 out of every $100 spent in America. The $161 billion total is less than the amount paid to the health and insurance industries, but education is far and away the biggest in terms of public involvement. Three out of every 10 Americans, or 62 million persons, are directly involved with education, either as students or as teachers and administrators.

Yet that staggering number is causing problems because it is so low.

As everyone knows, the baby boom is over. The 58.4 million students trooping back to class this week will fill nearly 3 million fewer seats that were needed in 1975, according to the Department of Health, Education and Welfare. But educators caution that this will definitely not automatically bring less crowded classrooms, richer curricula and glossier school buildings. On the contrary.

Everyone also knows about inflation, but somehow, the educators complain, the public has forgotten to apply that knowledge to the schools.

"The cost-cutting mentality is still very much with us," said Tom Shannon, executive director of the National School Baords Association. "People who think fewer students mean lower costs forget inflation and forget that the decline is spotty, a few kids here and none there . . . ultimately if you lose 100 kids in each school you have to lay off teachers and close the building, but until you do that, the costs are about the same."

And costs are up. Parents guessed per-pupil costs in public schools at $1,200 in a recent Gallup poll, but the real price is $2,100 or $11 per day, (HEW) says. For a handicapped child the average cost is twice that.

Yet the federal government provides only $218 more towards the education of a handicapped student than it does for any other. Administrators say mandatory education for the disabled is a long overdue, but they want help paying for it.

It is the same story in college. If tuition, room and board cost $3,500 at Princeton University in 1969, the same then as a new car, they cost $7,800 now, still the price of a new automobile. But somehow more people are outraged when education prices rise.

The pinch means that one-third of the nation's small colleges are in financial trouble, according to the Committee for The Advancement of Small Colleges. Larger or more prestigious schools are less likely to be pinched by student enrollment decline, but still worry about the cost of quality.

"It's hard to maintain libraries . . . a well-equipped chemistry laboratory used to cost $10,000 in 1969 and will run $1 million now," said Thomas Bartlett, president of the Association of American Universities. Faculties stagnate when hiring stops, and an older population with fewer children in school takes less of an interest in education in general.

Teachers cutbacks are the most tempting target for the institutions feeling squeezed, since fully 85 percent of most school budgets are salary costs. "When you talk about decreasingl spending, you're talking about cutting salaries," said Shannon, "but inflation has hit teachers as hard as anybody."

Strikes over salary issues in elementary and secondary schools involved 15,000 teachers and 415,000 students in seven states last week, with more expected as other schools open. "Teacher salaries peaked in 1971 before the wage-price freeze that year and have been losing ground ever since," said Willard McGuire, new president of the National Education Association, the nation's largest teacher union.

The average sub-college teacher earns $15,000 per year, ranging from $9,511 in Cincinnati to $22,000 in high-priced Alaska, the NEA found. These regional differences can be important.

Only some areas are losing students. The far Northeast, New York, Rhode Island and Washington, D.C. school enrollments are down, but numbers are steady in the north-central states and gaining to the point of overcrowding in some areas of the Sun Belt, which stretches from southern California to Florida

The school population follows the general population shift, but more than 30 states have enacted some kind of tax cut. There is hardly a school district where educators fail to report difficulty getting bond issues passed, budgets raised or salaries boosted. More than an enrollment drop is at work.

"The real reason is a general decline in public confidence in education," said Assistant HEW Secretary Mary Berry. "No matter how you cut it, it is true that test scores have been going down . . . and the public believes test scores are an important measure of student achievement.

They are not the only measure, according to McGuire. "The inference of these critics is that at some other point in time, everyone could read, write and compute," he said. "That's not true."

Only 15 percent of the nation's 14 to 17-year-olds were in school in 1910 and only 64 percent in 1943, but 95 percent of those youths are in school today, HEW figures show. Those who would have been dropouts or illiterate in an earlier age are sitting in classes, perhaps bringing the test averages down but raising the overall level of education.

Many more people now go on to many more kinds of higher education as well. In 1910, there were 52,300 students in 951 colleges and universities, but by 1940 there were 1.5 million in 1,700 colleges.

This year there will be 12 million in 3,095 junior, community, technical and military colleges and universities, public and private. That excludes the estimated 4 million non-credit students or those taking college-level courses at secondary schools, churches, their homes or in senior citizen centers. How does one test the value of their learning?

The tests themselves are under attack as the last of the baby boom enters the job market and finds the tests are labels that are hard to wash off. New York and California have new "truth in testing" laws requiring students to get their corrected and graded tests back and requiring testing organizations to disclose their methods, research and funding. Similar legislation is pending in Congress, but educators are wary of anything that could bring federal interest in curriculum matters.

The public worries about curriculum seem centered around the belief that there is too much of it, too much folderol.

"There is a cry to go back to basics, but we see little evidence that the basics were ever left," McGuire said."It's just that the schools are being called on to do many more things than they were a generation ago."

Parents want higher quality teachers, higher academic standards, tighter discipline, more personal attention for pupils (that is, smaller classes) and cheaper schools, all at the same time, a recent Gallup poll showed.

"I suggest," wrote Chicago Urban League President James W. Compton recently, "that the overriding goal for the '80s be organizing schools for the purpose of eradicating structured differences in opportunity and achievement among Americans that are a result of institutionalized practices for racial discrimination."

Desegregation is a continuing issue in many northern schools this year.

In Columbus and Cleveland, Ohio, and in Kansas City and Chicago, busing plans involving well over 100,000 students are in various stages of court battle. There may be more tests in Los Angeles and elsewhere.

Public schools continue to worry about "white flight," the move to the suburbs by white families who say they are scared away by discipline problems in inner-city schools. Many middle-class black families are joining the outward flow, causing inner-city schools to be the hardest hit by the enrollment decline.

Discipline emerges in poll after poll as parents' main worry. A National Institute of Education study found that one in every 100 teachers can expect to be attacked each year, in rural areas as well as in cities; and that one in eight will have something stolen or damaged.

"These are students who aren't disciplined at home . . . but the public expects them to be disciplined when they get to school," Berry said. "It's easy to blame the schools for the problems of society . . . and that's what happens when you try to get more money for the schools."

The result of all these pressures is that more creativity is being asked of educators than ever before. "We're making teachers be managers and they weren't trained that way," said Shannon of the School Boards Association.

Colleges competing for the dwindling number of high school graduates have hired public relations firms to improve their images. They go after promising students with leaflets, offers of financial aid, personal visits, advertising.

American University in Washington launched a "Jog Your Mind" campaign that boosted its continuing education program attendance significantly. There are schools for fundraisers offering courses in deferred giving plans and corporate gift solicitation. There are drives to woo foreign students and attempts to tailor course offerings to fit whatever students want to learn.