In the past year I have come across some good resources for teaching students lessons about being aware of their digital footprints. One of those resources is Frontline's documentary Growing Up Online which every parent and student should watch and can watch for free on the Frontline website.
There are two good slideshow presentations about digital footprint awareness on Slide Share. Sacha Chua's slide show The Gen Y Guide to Web 2.0 at Work should be seen by high school students. I've embedded that slide show below.
The Gen Y Guide to Web 2.0 at Work
Dean Groom also has a good slide show about digital footprints that teachers and administrators should watch as it stresses the importance to teaching digital footprint awareness. Check out Dean's slide show here.