Every once and a while you come across a federal government program that appears to be staggeringly large and perform an extremely useful service, yet you never hear a word about it.

This one is now within the Department of Education and people there told me it has been operating since 1958. It provides an interesting way to repay the federal money a person has received under either the National Defense Student Loan or National Direct Student Loan programs.

According to the Oct. 14 Federal Register (page 68004), a borrower can cancel part of all of his loan and the interest on it through full-time teaching at one of the nation's public or nonprofit elementary and secondary schools where the Education Department has found a high concentration of students from low-income families.

The formula is fairly simple. For those whose money came through the earlier "Defense" program, each teaching year can be used to cancel 15 percent of the loan. For the "Direct" program, each of the first two years cancels 15 percent, the next two years are good for 20 percent each, and the fifth year can be used out to wipe out the final 30 percent of the loan. Wiping out the debt comes on top of whatever the individual receives as salary.

The announcement of the 1980-81 academic program caught my eye because the lists of schools filled no less than 322 pages of the Register. They number, a department spokesman told me, more than 50,000, or almost half the nation's roughly 110,000 elementary and secondary schools, and represent schools where "enrollment of students from low-income families exceeds 30 percent of the total enrollment of students," according to the notice.

Given the millions of individuals who have received these federal student loans and the billions of dollars borrowed, you'd expect this loan cancellation program to have been quite extensive. As far as I can tell, however, it has not -- at least not according to the information the Education Department has received.

Of course, the information here in Washington is not too good because of the way the program is handled. Teacher-borrowers, it seems, report to the schools from which they obtained the money that they are in the program. The schools then seek from the Education Department the funds they would have received had the borrowers repaid their loans rather than participate in the teaching program.

The Department of Education knows only how much money it has paid the schools in the program -- about $90 million over the past 22 years. The guessing is that about a million people have taken part.

When the elementary and secondary education programs were reauthorized this year, Congress gave permission for individuals who teach school to get the same loan cancellation benefits. Let's see if more of them take advantage of the opportunity.