On the second floor of a squat, suburban office building, at the end of a corridor, the sign says merely "Foreign Medical School Services Corp."

Since the recent Grenadan military coup, six telephone lines have been in use almost nonstop, for this small office with its Naugahyde sofas and wall photos of tropical isles is U.S. headquarters for the St. George's University School of Medicine.

The school, founded in 1977, has been caught up in the invasion of Grenada, and the evacuation of its students was one of the principal reasons cited for the military action.

The institution was the first of a handful of controversial Caribbean-based institutions that have sprung up in the last five years in response to an unprecedented surge in medical school applicants. Each year 35,000 people apply to 127 American medical schools and 17,000 are rejected.

St. George's, with two campuses on Grenada, has a reputation of being the best of the group, although, as a for-profit institution with limited facilities, it is held in low regard by the Association of American Medical Colleges and much of the U.S. medical establishment.

Nonetheless, 84 percent of its graduates in 1981 passed the test required of foreign medical doctors here, administered by the Educational Commission for Foreign Medical Graduates. Only graduates of Tel Aviv University in Israel had a better pass rate, while the University of Guadalajara, Mexico, a traditional haven for Americans rejected by U.S. schools, had a pass rate of 49 percent.

St. George's founder is Charles R. Modica, 37, a lawyer who originally wanted to be a doctor but was rejected by 30 American medical schools and dropped out of the University of Oviedo medical school in Spain after a year. Modica reportedly rounded up a group of wealthy physicians, several of whom had children who had been rejected by U.S. medical schools, to invest in the project.

The school's vice chancellor is Dr. Geoffrey Bourne, retired from Emory University, and the father of Dr. Peter Bourne, a former adviser to President Carter. Scholars from Harvard and England's Royal College of Physicians have lectured there and the school has attracted many students with respectable grades from top U.S. universities who could not get into highly competitive medical schools in the United States.

Elizabeth Nelson, 24, a Princeton University graduate from Woodmere, L.I., who studied biomechanics in graduate school at the University of Maryland, said, "Educationally and medically it's a fine school. We had every possible advantage, lacked nothing in regard to the basic sciences we were studying." St. George's costs about $12,000 a year, including room and board, less than most private U.S. medical schools.

Along with several other students, Nelson said she was disappointed in the attitude of the school administration during the crisis. Her parents called her Monday morning and told her to come home. At 9 a.m., the student body assembled in the auditorium where a U.S. diplomat from Barbados said anyone could leave who so desired. About half of the 650 students filled out forms for departure, she said.

However, about 2 p.m., Bourne held another assembly. "He tried to make us feel guilty about wanting to leave," Nelson said. "He said if you all leave, the school will close down. . . . He said it was no different than the revolution in 1979 when only one man was killed . . . . He was telling us we shouldn't worry."

Efforts to reach Bourne through his son and through the Bay Shore office today were unsuccessful.

In the waiting room of the Charleston Air Force Base last night mimeographed copies of a statement were distributed to students from Modica, who had, all through the crisis, sent reassuring messages from the Bay Shore office.

The statement acknowledged that he had been wrong in believing Grenadan military authorities that the students could be safely evacuated by chartered planes if the need arose. "I realize my work was an exercise in futility . . . . I leaned over backward to keep good relations with the temporary leadership in Grenada."

The concluding sentence of Modica's statement drew bitter laughs from the students, many of whom said they would not return to Grenada. "After a short recess, we shall begin classes again," it said.