Found on the internets:
A professor stood before his class of 20 senior organic biology students, about to hand out the final exam.
“I want to say that it’s been a pleasure teaching you this semester. I know you’ve all worked extremely hard, and many of you are off to medical school after the summer. So that none of you gets your GPA messed up because you might have been celebrating a bit too much this week, anyone who would like to opt out of the final exam today will receive a “B” for the course.”
There was much rejoicing amongst the class as students got up, passed by the professor to thank him, and signed out on his offer. As the last taker left the room, the professor looked out over the handful of remaining students and asked: “Anyone else? This is your last chance.” One more student rose up and took the offer.
The professor closed the door and took attendance of those students remaining. “I’m glad to see you believe in yourselves,” he said. “Each of you gets an A.”