Phew. Done. And I didn't use any knowledge that a high school kid shouldn't have. Did I miss something?
One thing. How long it takes most people to understand a concept. It took me much more than 15 minutes to understand what a function is in high school, and I have spent far more than 15 minutes explaining it to other people. And it definitely took more than 5 minutes to understand the concept of a recursive function.
And you are assuming they have knowledge of concepts like string and whether or not they are printable...