IT IS KNOWN by appropriately coarse phrases at more than a few colleges and universities. In some cases, according to higher education's Change magazine, it's commonly called "an A for a lay." That's when an instructor offers a good grade or recommendation or other reward in return for sexual favors from students. In other instances, when punishment is threatened, it often goes by the alliterative "f--- or fail."

It's the kind of thing an Arizona State University student reported in a study at that campus in May: "One of my professors told me, 'If you'd be willing to get involved in some extracurricular activity, it could improve your grade.' I asked him what kind of activity he meant. He said to meet him at his apartment at 8 that night and I'd find out."

It's also often the threat of retaliation inherent in a professor's propositioning or fondling or making other unwanted advances toward a student. "Many professors simply don't understand the effect on the student of what they do, that she is frequently mortified by it, but afraid she will offend the man who controls her grades or career," says Bernice Sandler, director of the Association of American Colleges' Project on the Status and Education of Women.

But whatever you call it, explicit or implicit sexual manipulation of students by faculty or other staff is becoming an increasingly visible and vexing issue on the nation's campuses, one filled with complexity and paradox, at least once you get past the jokes about faculty fringe benefits and the student who complained of ending up only getting a D from the lover anyway.

The jokes don't help much with the nasty bits of evidence emerging from some campuses, the grievance procedures being instituted, the charges and countercharges filed, the handful of professors already disciplined, the fear of misunderstandings, of malicious accusations, of wrongly damaged careers, of professors consequently staying clear of female students -- and of that, too, shortchanging women in their educations.

Start with the bits of evidence. The Arizona State study, for example, found nine students, or about 2 percent of the student respondents, saying they had faced bribes or threats from instructors for sexual activities. That may sound like a mere nuisance to some, just 2 percent. But if you were to apply that to the total female college student population of roughly 5.5 million, you would happen to get 110,000 students.

Mary Rowe, special assistant to the president of MIT, has been dealing with the problem for nine years there -- about 25 MIT student grievances a year, she reports -- and helping numerous other campuses. She estimates that, indeed, "at least 2 percent of any female student population will run into some fairly serious problem of this kind in any given year, and an additional significant but much smaller group of men. The male students usually are also concerned with male offenders." The Association of American Colleges' Sandler notes that, additionally, there are "some instances of male students being propositioned by female professors."

Nancy Carlson, director of counseling and career services at the University of Rhode Island says "there are probably somewhere between 10 and 100 cases a year" of these types there. A 1980 Rhode Island survey actually turned up three students who reported being propositioned themselves in return for grades or test marks and 39 reporting similar propositions made to other students at the campus. Two students in recent years have gone so far as to take their complaints to the administration.

"In one case last year, the faculty member resigned when we brought it to his attention," says Douglas Rosie, assistant vice president for academic affairs.

You can find other scattered examinations of the issue with wider-ranging definitions of offenses and thus more of them reported. But perhaps the best comment on the extent of the problem comes from Jerold Roschwalb of the National Association of State Universities and Land Grant Colleges, who simply states: "There is more of this going on than many people believe and less than others fantasize."

Mutually desired professor-student sexual relations, of course, have long been common, especially at the graduate school level. A 1979 survey of the American Psychological Association's psychotherapy division members, in fact, found that fully a quarter of women respondents who had received their doctorates in the preceding six years had engaged in "intercourse or genital stimulation" with their professors. Nobody believes all of that was coerced.

There are also, to be sure, women students who are eger or willing to sleep with professors in return for academic benefits, particularly with today's intense competition for the right medical, law, business or other graduate school. Prof. Richard Taylor at the University of Rochester reports in his Change magazine article that "to many students this practice seems to be little more than an offer of one favor in exchange for another. As one of them expressed her own attitude, 'I've already lost it, once more won't make any difference -- and I'll get an A in Bio.'"

That's not the kind of student achiever you'd want at your university, of course, and you've got to worry about any professor who goes along with selling favoritism for sex, no matter what lure or "midlife crisis" he may be facing. As MIT's Rowe aptly states of the overall problem, "It is most fundamentally an abrogatin of the contract we have to lead, to teach, to inspire, to foster excellence."

The problem for the couple can also come after the affair breaks off. In one case with echoes of that one-liner about the student who got a D anyway, a former Indiana University student in speech pathology filed suit several years ago after her ex-love/professor outright flunked her. She claimed he acted maliciously, out of spite. But in March 1979 the federal district court judge dismissed the case, unconvinced from her academic work that the instructor had acted improperly.

However one feels about voluntary cases, though, there is no doubt that there's a serious problem in professional coercion of sexual favors, or that until recently there was little outcry about it. In part, this was because other faculty members or administrators who knew of such cases were hesitant to act.

Jane Levin, a clinical associate at Washington University's Graduate Institute of Education in St. Louis, for example, was well aware of the problem. In the past four years, she says, three students came into her office "telling me that a male faculty member either threatened them with punishment or promised them a higher grade if they would have sexual relations with them."

But she adds: "Quite franklin, I didn't wnt to know the details. If I knew who it was and more about what had gone on, I would have felt compelled to intervene. That would have been very difficult, and perhaps with negative consequences for my job. I do not have tenure." Levin is now part of a group working to sensitize her campus to the problem.

In large part, though, all the reports suggest, the invisibility of the problem was -- and stillis in most cases -- due to the students' fears of embarrassment if they reported the episodes, to their sense of shame or intimidation or self-doubt, to worries -- evidently well founded at some campuses -- that nothing would be done anyway. Better just to try to void the professor, change courses or even majors or otherwise handle it yourself.

Some young women do indeed handle it, and rather nicely. The Arizona State student who reported being propositioned for "extracurricular activities," for example, stated: "I told him to go take a flying leap and if he ever said another word or changed my grade to something I didn't earn (lower or higher), I'd report him."

"Good for her," says the Association of American Colleges' Sandler. "But unfortunately many other students fear that they wouldn't even be believed, that it would be a young student's word against the respected scholar's, and so they are just unwilling to report it. Reporting sexual offenses by your professor -- or what the student takes as such an offense -- is a very difficult step for most students, and certainly one which nobody should take lightly. That's why specific policies and sensitive, confidential grievance procedures need to be established in this area on all campuses -- for the sake of both sides."

You will hear variations on that theme from almost anybody you talk to who is groping with this issue: the need for policies and procedures, incorporated in or added to long-existing student and faculty grievance processes. It doesn't seem like much to ask. Grievance procedures for sex discrimination complaints in education are already required by federal regulation, under Title IX of the 1972 education amendments for campuses receiving federal aid. But that doesn't mean most institutions have done much, if anything, about it.

In fact, only a handful of campuses in recent years -- Ohio State, Rutgers, Stanford, Yale, the University of California at Santa Cruz, Brown, and the University of Washington, among others -- have taken such action.

This has come chiefly in the wake of a widely noted Yale case, Alexander v. Yale , in which a student four years ago charged a professor had unfairly given her a C in a course after she refused a sexual proposition by him. The federal appeals court last year held that the student failed to prove the damage she claimed, and that Yale in any case had already established the grievance board her suit had sought.

"During its first two years, the board received three signed complaints from students about what they thought might be instances of sexual harassment," says Yale Associate Dean Judith Berman Brandenburg. This was in addition to an unspecified number of students who came to discuss incidents but did not sign complaints. Of the signed complaints, Brandenburg adds, "In one case the matter was concluded by a discussion arranged and conducted by a board member between the student and the faculty member. The other cases were concluded after two board members intervened."

As with others, Brandenburg stresses the gulf that often exists between student and faculty perceptions of an episode. "A student may consider someone's remarks or actions terribly frightening or coercive," she says, "while the person on the other side may actually considers the words or actions as a compliment and may not be aware of the powerful unintended effect on the sutdent."

On other professor-student sexual cases, as with the University of Rhode Island professor who resigned, the consequences for faculty have been more severe. Prof. John Goheen, ombudsman at Stanford University, tells of "three or four cases" reported to the administration since 1978 where sex was solicited from a student by a faculty member. One persistent proferssor in the sciences "was reprimanded and given a salary reduction," he says.

San Jose State fired a philosophy professor in a case in which five students had accused him of fondling, propositioning and embracing them. Harvard disciplined a noted government professor for advances to a student in his office. The University of California at Berkley suspended a sociologist last year for one academic quarter in a case in which a dozen students had charged the professor with sexually harassing them.

In the Berkley cse, the university said that "it appeared that some of the alleged misconduct was in itself minor or the circumstances ambiguous," and that "no complaintant suffered direct academic injury from his action." Nonetheless, it found the suspension was warranted because of the sociologist's "serious departure from academic behavior."

On the other side, there have been unpleasant consequences for students who have become involved in sexual charges against a professor.

Most notably at Clark University in Worcester, Mass., after a female anthropologist accused a male sociologist of sexual harassment, two women graduate students gave their own evidence against the man at a special committee hearing. The case took complicated twists and turns, with the man's supporters, among other things, charging that he was really being attacked because of his activist politics. The most recent turn: The sociologist filed a $23.7 million lawsuit in May against the woman faculty member, the two graduate students and two other women who testified against him at the campus.

Clearly, there is enough here to produce great caution on all sides of the issue. But paradoxically, this is an instance where caution itself in some ways becomes a feared consequence -- particularly where it might cause professors or students to treat the others more formally and coldly and sour the normal relations essential to education.

In the Yale case, for example, a male classics professor who was among those joining the women student's suit said that faculty members' "professional effectiveness in teaching and in engaging in the pursuit of knowledge with students is seriously impaired by the contamination of the student/faculty relationship created by tolerance of sexual pressures, which . . . generates an atmosphere of distrust unconducive to teaching and learning."

His charges were dismissed, and Yale is among the few campuses with formal policies and a grievance procedure now to deal confidentially with such complaints. But his point is well worth keeping in mind.

This is especially so for the small number of professors who may abuse their authority by sexually propositioning students, for campus officials who understand their obligation to both pursue legitimate complaints and head off misunderstandings, and for women students who may be pondering whether to file complaints.Important as their own circumstances are, something far broader, potentially affecting large parts of university life, is at stake.