During exams, students are left alone in their classrooms without proctors, or allowed to take the questions home. Dormitory rooms are generally unlocked, and to get into a football game a student need only have a friend with an identification card vouch that he is enrolled.

At the University of Virginia, the honor code governing student behavior is as much a part of the school as the Lawn and the man they call Mr. Jefferson.

For those who break the code by lying, cheating or stealing, the punishment is simple--permanent expulsion with no appeal to administrators and no relief in the courts. A committee of students, deliberating in a low-ceilinged room with a bust of founder Thomas Jefferson, makes the decision.

Opponents have argued that the penalty's very strictness makes the honor code hard to enforce, but for years they were unable to add any alternatives to the "single sanction" of expulsion for dishonor that has marked university life here since 1842.

Today and tomorrow, however, the university's 16,000 students are voting in a referendum on whether to soften the penalty for first offenders, after an often-emotional campaign debating honor and punishment and the value of the school's many traditions. If the change is approved, there will be an alternative punishment for a first-time violator--a one-year suspension.

The campaign has included television ads, posters, forums, and even a post-midnight appearance by members of a university secret society--six masked students in purple robes called the Purple Shadows. The Shadows turned up in dormitories Tuesday night, silently slipping cards under bedroom doors.

Their message, printed in purple ink, opposed the end of the single sanction, which they say has been essential for the honor code to function and create a "tradition of mutual trust . . . in Mr. Jefferson's academical village."

"That's nonsense," said John Snyder, a 22-year-old senior from Alexandria who is heading the campaign to allow suspensions. "It's a myth that if you go to the University of Virginia you don't lie, cheat, or steal. It merely means you have not been convicted . . . .

"The cheating goes on and most students turn the other way," Snyder continued. "They're not willing to accuse somebody and have them leave the university and ruin their life. If we had suspensions , the system would be better enforced."

"We have a system now which works well," rejoined Nancy Lyons, 21, an engineering student from Arlington who is chairman of the elected 11-member student honor committee. "I know we have a very unique system, and it's not perfect. But we have a community down here where cheating is not common. We have a comfortable atmosphere where students and faculty can work together without suspicion.

"If [honor code violators] don't choose to function that way, they should leave," said Lyons, who has a coveted room on the Lawn, the Jefferson-designed centerpiece of the university. "Letting them be suspended for a year and then come back would just weaken that emphasis on honor."

During the past 15 years the size of the university has more than doubled and the honor system itself has changed--with dozens of detailed procedural rules replacing more informal justice. Still, the system remains one of very few of its kind at American colleges, with both accusations and convictions made solely by students. Its impact remains substantial, for the code extends well beyond tests and academic life.

For example, in lecture halls coats and bags are often left by the door.

Merchants on University Avenue, the main business street, take students' checks from out-of-town banks with no questions asked, and even at a cafeteria many students pay with a check for their meals.

"The university is no longer a southern men's aristocratic institution," said Joe Blanton, 22, of Sikeston, Mo., a leading foe of the sanction change. "It's a high-pressure, competitive academic place. But it's still the world's only publicly supported private university. And we want to preserve that old U. We don't want it to be turned into SUVAC--a State University of Virginia at Charlottesville."

Still, during the past few years the university library, like many around the country, has installed security gates with an alarm system because of lost books. The athletic department has stopped allowing students to vouch for each other at racquet-ball courts and the swimming pool because too many outsiders were let in; instead nobody is admitted without an ID card.

It is virtually impossible to gauge the extent of cheating, but a confidential Gallup poll taken last year for an honor-system study committee found 3 percent who acknowledged they had cheated on an exam. Ten percent said they had seen other students cheat but had not reported them.

For the sanction change to be approved, the referendum requires a three-fifths vote. During the past 10 years, six other proposals to end the single sanction of expulsion failed, but the code has been softened in other ways.

Traditionally, advocates of the code boasted that there were "no degrees of honor," but since the mid-1970s, an honor panel must find that an act was "reprehensible," not just that it happened, in order to convict. The traditonal two-page "blue sheet" describing the system has been augmented by a constitution and 50 pages of by-laws. Under the detailed new rules, testimony is allowed on the "psychological condition" of the accused, and "sincere retraction" before being charged with a transgression is a complete defense.

The accused can also ask for a jury that includes randomly selected students instead of just honor committee members, and can request a public trial, though almost all hearings are still closed.

Last year, the first time randomly selected juries were used, only three students were convicted out of 14 brought to trial, though so far this year 4 out of 6 have been convicted.

Over the last two decades, the average number of students dismissed each year has stayed at about 10--although last year it dipped to seven, including voluntary withdrawals.

"The honor system is being straitjacketed by all the procedural considerations even though severity of the sanction has been preserved," said Harry F. Gamble Jr., a religion professor who headed a faculty study of the honor code. "We're looking for a more active prosecution."

Administrators and faculty members have kept hands off the student operation of the code. But last year a joint student-faculty committee recommended that a one-year suspension be allowed. At the same time it urged making it easier to convict by dropping the test of reprehensibility and the defense of retraction.

The referendum proposal would only reduce the penalty. But Gamble sees it as a step in the right direction.

"There are many noble ideals in the honor system," he said. "The question is whether we can afford so much nobility at the price of realism . . . Maybe if the sanction is moderated, students will be less reluctant to proceed."