Johnny's dad had told Johnny that if he could get an A+ on his final exam, he could get any ice cream flavor he wanted plus a pizza. When the day for the final exam came, the professor said, 'There are three questions on this exam... -Riddles
When the professor confirmed Johnny's anonymity (the professor didn't know who he was), Johnny quickly slipped his exam paper into the pile of exams and ran off, so the professor wouldn't know which exam paper deserved an F. Congratulations on sticking through the whole riddle.