What's the worth of an MBA?

Is a sheepskin that costs you $21,000 in tuition and another $15,000 in room and board - and, for many, a two-year office sabbatical and salary loss - really worth it?

That's what today's ladder-climbers are forking out for full-time master-of-business-administration (MBA) programs at Wharton, Harvard, or Stanford. But then there's the revenue side of such an outlay: average starting wages of $40,000 a year.

Despite corporate grumbling about overpriced, unseasoned executives, a booming economy has brought recruiters back on campus en masse.

''It was an excellent year,'' says Roger Muller, placement director at the Dartmouth's Amos Tuck School of Business Administration in Hanover, N.H. ''The average number of offers per student was as high as in any of our very good years.''

And starting salaries are getting higher, too. Many of the top schools report about an 8 percent increase over last year.

The past decade has been, quite simply, a raging bull market for business schools. In 1974 there were 389 schools turning out 32,820 MBA grads. In 1984 there were about 600 schools producing roughly 64,000 grads.

Yet every bull market has its corrective phase, and for the past two or three years pundits have said an upheaval in business schools is imminent.

''When the market turns, as I believe it will, the world will not be kind or very tolerant of those educational institutions that fail to meet real needs other than their own,'' the dean of Harvard Business School, John H. McArthur, warned in 1980.

The reasons for such pessimistic prognostication?

* The pool of college graduates is shrinking. Demographic trends show a dip of as much as 30 percent over the next decade.

* Tuition has increased. As the cost of going to B-school rises, prospective students are thinking twice about the potential benefits.

* The number of women entering MBA programs has peaked, observers say. ''The most significant contribution to the growth has been women entering management, '' says Kenneth R. Smith, dean of the College of Business and Public Administration at the University of Arizona, Tucson. Today, women account for one-third of all MBA degrees conferred.

* The number of would-be business students taking admissions exams is slipping. The Educational Testing Service, Princeton, N.J., reports that in 1980 , those taking the Graduate Management Admission Test peaked at 216,000. Since then the numbers have declined about 7 percent each year.

But the ''shakeout'' still appears to be somewhere over the horizon. A check with several schools shows the number of applicants had been declining for the past two or three years, but this year the decline appears to have stabilized.

''It's hard to know what is the proper interpretation of application figures, '' says William Broesamle, president of the Graduate Management Admission Council. He suggests it may be that students are applying to fewer schools, becoming more selective.

Even if applications should continue to decline, many schools still have anywhere from five to eight applicants for every seat - a sizable buffer to dissolve before they feel the pinch.

As for the shrinking pool of college graduates, Colin Blayden, the dean at Tuck, notices an offsetting trend at his school. ''We've found the characteristics of our (applicant) pool are shifting. There is an increase in older applicants with more work experience.''

In fact, the working world, not the undergraduate campus, may prove to be an increasingly important source of students.

The corporate connection is already seen as a valuable asset for any school. Companies provide jobs for graduates, a source of revenue for research, and they can enhance program quality. But it's not a one-way street.

''The corporations benefit from easier and earlier access to the product as well as from collaborative research efforts,'' notes Mr. Smith at Arizona.

And the corporate demand for the B-school's services is there - especially for part-time MBA programs and short advanced-management courses.

''An estimated $30 to $40 billion is spent each year by corporations on executive training,'' says Charles Hickman, associate director of the American Assembly of Collegiate Schools of Business, St. Louis.

A decade ago there were 10 schools offering ''executive'' MBA programs. Now there are 92, and the numbers are increasing. The EMBA is a part-time program designed for mid-30 or mid-40-year-old corporate types, perhaps engineers or production-line managers, who are looking to move into upper management. The course is paid for by the company, and the student continues on his job, perhaps missing one day every other week. Typically, the classes alternately run all day Friday and all day Saturday.

And the market for management education is growing, Mr. Hickman says. ''Management needs to know about things (computers and telecommunications) that weren't even subjects five years ago.''

''Much of the growth will be in non-degree programs,'' Mr. Hickman says. ''There, the schools will be competing with in-house corporate training, nonprofit outfits, and profit schools. It will be interesting to see how they respond. Traditionally, business schools have been slow to respond.''

If and when a shakeout does occur, many do not expect the top schools to be hurt. Not surprisingly, at the University of Pennsylvania's Wharton School, vice-dean Anthony M. Santomero holds this view. ''We'll probably enter a phase similar to that which has occurred in other product lines: A few highly visible brand names, followed by many generic products.''

Hickman agrees to some extent: ''The well-established schools are well supported. If there is a fallout it will probably follow 'last-in, first-out' lines. The new entrants will go first, unless they can find a niche.''

North Texas State University is a school that has found a niche: It is now considered the place for petroleum accounting, Hickman says.

At the University of Arizona, the school's strength lies in its Management Information Systems program. Dean Smith calls it ''one of of the four or five best in the US.''

For shallow-pocketed students, state-supported business schools do have an edge. For instance, the University of Arizona's annual in-state tuition is $900, out-of-state is $3,800. But then, the average starting salary for graduates is lower, too: $26,500.