If you're looking for the most popular course among college administrators these days, try taking Demography. There, in among the statistics, you can find all the ironies in the current pitch for academic excellence in higher education.

To put it simply, colleges are being asked to raise their demands on students, especially admissions demands, at the exact moment when their supply of 18-year-olds is diminishing.

We now have the College Board beefing up the guidelines for college-bound students in an agenda called "Academic Preparation for College." We have assorted public universities setting new admissions standards. We have a national dialogue about striving. But we also have the Census Bureau telling us that by 1990 the number of 18-year-olds will fall from 4.19 million to 3.4 million.

All this adds up to a very real conflict for colleges, especially private colleges, trying to respond to the desire for excellence and the desire for survival. As Ernest L. Boyer, the president of the Carnegie Foundation for the Advancement of Teaching, says, "There is no question that the urgings toward higher standards are on a collision course with the reality of both demography and financing."

Colleges and universities are now faced with these kinds of choices: does a school with fewer applicants cut back on its potential students? Or does it change, like any other business, to attract more consumers?

There are a variety of responses to the crash course in Demography. At the upper end of the academic scale, the competition for the best and brightest has heated up. Last week Brandeis joined a growing list of schools awarding scholarships on the basis of merit rather than need. The Ivy League now woos high-achieving students the way football leagues woo high-scoring players.

At the same time, a few colleges have made a commitment to quality over quantity and decided to actually cut enrollments. This year Smith College said it would shrink future freshman classes by 10 percent. Ohio-Wesleyan started a "Reach for Quality Program" --one reason why its freshman class went down from 662 to 488. Last week the president of Gettysburg College announced that the college "will not allow its quality to suffer by enrolling students who do not meet its selective standards."

But it's a rare school, and often a well-endowed school, that would take such a stand. Cutting back on students means cutting back on income and on teachers. In real life, slashes are usually forced on institutions, not chosen by them.

A college is more likely to become engaged in a competition that mirrors any other business in hard times. It's estimated that 200 to 500 colleges will fold in the next decade, but not without a struggle. We are already seeing schools change their image, and their curricula, the way 7th Avenue changes fashions to attract the consumer.

George Keller of Baltimore's Barton-Gillet Co., one of the new strategists hired by colleges, says: "We get called in to worry about beefing up the marketing and soliciting of students, and stealing good students from other clients." The reality is that we're already in a buyer's market. Whatever our mystique about higher education, only about 100 institutions are highly selective today.

While nearly all colleges maintain the illusion of an admissions hurdle, demanding SATs, records and interviews, a large number accept and recruit warm bodies. Given the choice between excellence and survival, these institutions choose survival.

In the coming crunch, the gap between the best and the rest is almost certain to grow. Carnegie's Boyer predicts that the colleges that are already in high demand--somewhere between 15 and 25 percent of all colleges-- will respond to the current urge toward excellence.

"The rest of the sector," he says, "will have less and less respect as credentialing centers in which the credentials mean that significant academic and intellectual achievement has occurred."

We've always had trouble thinking of college as a business and students as consumers. But high standards may be increasingly for the carriage trade. In the current climate, the academically rich institutions will grow richer. The rest may flunk Demography.