If you didn't know, Cleve has written two electronic books on using MATLAB for mathematics. The "Numerical Computing with MATLAB" book is also available in traditional print format. They come with a wealth of MATLAB examples, some of which are suitable for high school students (in "Experiments with MATLAB"). You were able to download the example code from their web pages before, but now, if you have R2012b, you can install the examples as an App with a single click! Here are a couple of my favorites from his examples.
Experiments with MATLAB - Orbits
Numerical Computing with MATLAB - Walker
Get the MATLAB code Published with MATLAB® R2012b