There is an old canard about professional economists: their job is to see if things that work in practice work in theory. So it is with education. What accounts for the quality of teachers? The economic and social realities of a given time and place. F. Scott Fitzgerald is reported to have said that nostalgia is being sentimental about something that never was. For those of us who graduated from high school before 1960, it is hard to distinguish between romantic memory and reality.

Were my best high school teachers-- Mrs. Nesbit and Mr. Cunnea, Miss Callahan and Mr. Dean, Miss Walters and Mrs. Roberts--really as demanding, and rewarding, as they appear through the mists of time? The question answers itself: the good teachers, the ones who really challenged you, forced you to think, are the ones you remember because they changed your life. And there are individual teachers like that today.

The more difficult question has to do with teachers as a group: were the teachers of 10, 20, 30 years ago "better" teachers than those teaching today? The numbers give one pause, for they paint a dismal picture.

Gary Sykes, a reseacher at the National Institute of Education and an authority on teacher quality, reports some sobering facts: "Graduate Record Exam . . . scores . . . have declined significantly since 1970, and were substantially lower than scores of majors in eight other professional fields." After analyzing data from a wide variety of sources, Sykes somberly concludes that ". . . a mass of evidence converges to show that academic ability of education majors is both low and declining. Teaching appears to attract the least academically able and to be decreasingly attractive."

The probability that yesterday's teachers were better than their successors is so plausible that various explanations are making the rounds. The most popular is that it is all the result of women's liberation.

The argument is by now familiar and goes like this: for generations, only one socially desirable job was available to women: teaching. The only other female employment options--scullery work, telephone operator, household domestic, chambermaid, and the like--acted as a negative incentive, encouraging the best and the brightest women to enter teaching, a public school bonus enjoyed at women's expense.

The most talented and ambitious women became teachers and they worked for very low wages. Indeed, for years the situation was so extreme that women teachers were actually on a separate salary schedule and paid less than male teachers.

What was true for women in general was also true for minority group members, particularly black women. It is an old story in the black community that one of the few jobs in which a black woman was secure was teaching: secure, at least, from the advances of unscrupulous employers in domestic, farm or factory service.

The halting emergence of equal employment opportunities for both women and minorities, then, has denied public schools a supply of low-cost, high-quality labor. That is only part of the story, however. Another equally important pair of events does much to explain why the overall quality of today's teaching force has declined.

The events in question are the Great Depression and the Wondrous Recovery that followed the Second World War. First, the Depression. It tipped untold thousands of highly qualified adults into teaching, a group that in more sanguine economic times would have done other things. By 1931, unemployment had reached 15.9 percent; by 1933, the absolute depth of the Depression, unemployment reached an incredible number: 24.9 percent. A teaching job was worth its weight in gold, and the competition for those that were available was intense.

There were not only no other jobs available, teaching was one of the few jobs a bright and eager person could prepare for at reasonable cost. During the Depression, for many, a two-year teaching degree from Normal U. was possible, while Harvard was financially out of reach.

All but unnoticed, an interesting scenario unfolded. The entering Depression teacher stayed in the profession because no lateral employment options were available. The interruption of the war cemented the situation firmly into place. Teachers who entered the armed forces returned to their old jobs with seniority and pension benefits intact.

Hard upon the end of the war came unparalleled recovery and real economic growth. Schools shared in this, and it was not long until the children of the baby boom swelled the schools' ranks. By the early '50s, in certain growth areas it was not uncommon for a school district to spend most of its energies building schools: adding a school building a year was not uncommon for a medium- size district in California, and even this did not prevent double shifts and overcrowding.

The effect of this for Depression teachers was quite dramatic: 20 years after entering teaching (by default) they found themselves committed by force of circumstance to their craft.

It is, indeed, an ill wind that blows no good, and the Depression produced a generation of teachers of exceptional talent and intelligence that we began to take for granted. We thought their high standards, intellectual accomplishment and dedication were the norm. But high standards for elementary and secondary school teachers have never been the norm: among the best, the pay was never sufficient to attract any but the most dedicated.

Today's good teachers--of whom there are still many--are in the classroom because they believe in teaching, not because of financial rewards. That, of course, is an old story: in the "helping" professions, "psychic" income is important. But schools cannot be run on good will and dedication alone.

It should come as no surprise to learn that as the Depression generation teachers began to retire, the slow slide in the test scores began. The young teachers who began in 1929 are now in their mid seventies; they began retiring in the 1960s. The teach