Students at Columbia University finished their exams three weeks ago.

Students at Yale University have also been done with exams for almost three weeks. And Brown University students finished their semester two weeks ago.

But some unlucky Harvard students are still pulling all-nighters and cramming for exams and won't be able to leave Cambridge until tommorrow.

In its calendar, as in so many aspects of day-to-day life, Harvard has clung to what administrators call a traditional path, with a fall semester starting late and exams after Christmas break, and a spring exam schedule that this year threatens to run into June.

Most Ivy League schools and some outside the exclusive cadre have switched to a new exam system, one that makes some frustrated Harvard students green with envy in late May.

But Harvard harbors no plans to change its system.

"There are constantly proposals for different calendars," says Margaret E. Law, registrar of the Faculty of Arts and Sciences. "Students want to go home for Christmas without the worry of studying for exams, but they don't realize what that change would involve," she says.

In order to leave school when most other college students do, students would have to give up some holidays as well as the luxurious two-week reading period, according to Law. "Any change in the current system would condense the semesters and make things more difficult for the students," she says.

Harvard's system is unusual, but not unique. There are still some schools--Princeton and New York University (NYU) among them--that have not changed over to the now-customary exam schedule.

Academic calendars became the subject of heated controversy on many campuses in the late 1960s and 1970s because students and faculty had conflicting views on what the best exam system was, says Columbia University Registrar Zeita-Marion Lobley.

Students usually wanted longer vacation breaks, while faculty members wanted the conventional system because it was better aligned with early September research deadlines and professional meetings, Lobley says.

In recent years, often after much debate, many colleges have changed to the new system, in which the fall term begins right after Labor Day and ends with exams before Christmas.

Typically, students under the new system have almost a month of winter vacation. They return to class in late January or early February for a spring semester without a long reading period ending in early May.

Brown, Columbia, Cornell, and Yale are among the Ivies that have changed to the new system. Other colleges, such as Dartmouth and Stanford, have instituted unusual trimester or quarters systems very different from the traditional calendar.

Administrators at colleges that have switched their calendar often sav that students and faculty have expressed satisfaction with the move. Others, however, say the new system has engendered mixed feelings among students who have felt more stress in the condensed academic year.