Regardless of their decision, all the students must retake their midterm exam this week.
Professor Richard Quinn addressed his students in a videotaped lecture, explaining that the test scores "were a grade and a half higher than [they had] ever had run before."
Those elevated test results sent up a "red flag," so Quinn ran more complicated statistics on the exam results. He said he then received confirmation of his suspicions when a student, "either through a guilty conscience or as a head's up," anonymously tipped him off.
Two hundred students, approximately one-third of the class of seniors, were believed to have received advanced copies of the exam. It was the largest cheating scandal in the university's history.
Quinn, who called the scandal "a knife to my heart, calculated exactly who'd cheated, and then gave the entire class a dressing down.
"To say I'm disappointed is beyond comprehension," he said. "Physically ill, absolutely disgusted, disillusioned, trying to figure out what the last 20 years were all about."
He offered the students an ultimatum: Come clean and take a four-hour ethics course, and your records would be wiped clean. If they chose not to come forward, they'd run a risk.
"If you want to take a high-risk gamble, take it. I challenge you to take it," he said. "Because we know who you are, we know where you are and when academic affairs is done, you'll know the outcome."
Aside from the ultimatum, Quinn is making all 600 students retake the exam, whether they cheated or not. He has given the cheating students until midnight Wednesday to come forward and take the ethics seminar to risk expulsion.
So far, he told ABC News, about half of the cheaters have confessed.
Students Split on Professor's Threat of Expulsion
The cheating was particularly surprising, because the exam was conducted in a cutting-edge laboratory that uses "anti-cheat cameras" similar to those in a gambling casino.
"It's hard for me to know how they would cheat, because there's monitors walking around the testing labs," said Samantha Riordan, 20, a student heath service administration major who works in advertising at the student newspaper.
"If this is your major, you should be studying," Riordan continued. "You should want to learn and not to cheat."
Student Alan Blanchard agreed.
"It's horrible," he said. "We don't need unethical people going into the business world, obviously. I'm sure there's enough of them out there."
But student Konstantin Ravvin expressed a different opinion, accusing the university of "making a witch hunt out of absolutely nothing, as if they want to teach us some kind of moral lesson."
"This is college. Everyone cheats, everyone cheats in life in general," Ravvin said. "I think you'd be hard-pressed to find anyone in this testing lab who hasn't cheated on an exam."
But morals are exactly what Quinn is hoping to teach his students.
"If they're going to learn one thing coming out of university," he said, "they're going to learn dignity and honor and the value of ethics and honesty."
In his lecture, Quinn thanked the students who did not cheat but made his feelings clear for those who did: "For those of you who took the short cut, don't call me. Don't ask me to do anything for you ever again.
"That's it. Those are the hours on your makeup exam. If you have to give birth, you'll give birth in the exam room," he said. "So adjust your schedules, blow off whatever you have to blow off to be there. This is one shot. ? If you miss it, too bad."
ABC News' Sarah Netter contributed to this report.