Change the wording to make the user feel special
Add authorization to Challenge.  Update the Challenge views
Add ChallengeUser model to keep track of users that finish a given challenge.  It's a many-to-many relationship so it can track when users have completed a challenge more than once.
Show the number of users that have completed each challenge.  The code is not efficient (n + 1 query problem) and inaccurate because it doesn't account for users that complete the same exam more than once, but it's something