I certainly have an appreciation for anyone who teaches, especially when teenagers are involved. Teaching about Illustrator can be thought of like a double edged sword when it comes to teenagers. On one side it is about math which can turn kids off. On the other side is art, if taught right, it can show the kids that math is beautiful. Therefore it could create a spinoff for some of the kids wanting to learn more about math.
Off the top of my head, I'd say have them either sketch or find internet samples of "graffiti style" lettering (something fun: the curvier, the better) that they could make they're own monogram tag out of. Then scan it and trace it with the pen tool (That's by far the most important skill to possess and it carries over into almost any other graphics program.) Once they have that, they can experiment by filling it with patterns, blends, styles, gradient mesh, whatever.
A few years ago there was this great little design competition called "Project Vormator" where you get a handful of basic vector shapes and are asked to demonstrate your creativity with them. The competition is long over (there's a book with top entries you can look at though) but it's still a great exercise. Students can apply transformations such as rotating, and scaling, apply strokes, colors and gradients, apply pathfinder transformations, and exercise their individuality and creativity at the same time.
Great for beginners in particular, I thought.
Here's a link: http://www.vormator.com/
Look for the teeny tiny little link to the original instructions and shapes on the bottom of the page.