In one of the pages from this topic. Twice. Search for leaderboard, etc. Read the topic. But if you really don't want to waste your time searching, then here goes a summary: I can't make a global leaderboard simply because quizzes are different in time to complete, number of questions, number of replies per question, etc. So I don't know how can I say that JOHN DOE that replied a 10 questions quiz, in 5:37 with 10 repliles per question is better than MARY DOE that replied a 3 questions quiz in 1:45 with 3 replies per question. So you, as the others did, will say: "Do a formula!". Ok, I couldn't make it work. If you can, let me know.
Leaderboard is displayed only in quiz page, the only POSSIBLE place to display it.
Once you complete the quiz, you got the RESULT, which isn't saved obviously, since it is a simple way to show whay you did right or wrong.