Exploring Visual Design looks good to - but that covers all design -
not just print/graphic.
This looks like a good book to buy yourself for refernce:
Teaching Graphic Design: Course Offerings and Class Projects from the
Leading Graduate and Undergraduate Programs
by Steven Heller (Editor)
Heller is a respected name in the design world.
I bet a lot of the lesson ideas can be stepped down to high school.
Here is another that I saw listed on several sites:
The Elements of Graphic Design by Alexander W. White
I found a lot of Graphic Design books... If you intend to do a lot on
computer - look for the most recent publication dates. I was curious
and did a search for recommended texts on TeacherArtExchange and the
only posts I found were from 96 and 97.