I'll start by saying how cool it is that I think you're doing this for your class. I wish someone had turned me on to the possibilities that entrepreneurship brings when I was in high school.
Hard to go wrong with some Seth Godin. Here's a link to Seth's best from the past two years.
http://sethgodin.typepad.com/seths_blog/2008/09/best-of-the-...
I also like this series from Mark Cuban called "Success and Motivation"