I've been learning Python with the help of Jason R. Briggs' Snake Wrangling for Kids, a free, online book that is targeted at younger students but which I find perfectly engaging, too. Mr. Briggs does a great job introducing students to the concept of programming with good examples, snippets of code, and a very logical, studied approach to learning to program.
OS X 10.5.x has Python included, though it is a slightly older version than the current 3.x release. However, for the exercises there is a slightly older version of the book available so you can follow along and get the code examples working just fine.
I've shell scripted in the past and learned a little Perl, so I'm pretty comfortable with the syntax and approach to programming using Python. If you are looking for a way to teach younger students how to program, Snake Wrangling for Kids might be a great starting point.