You can test your knowledge immediately using the Java tutorial pages at:

The Interactive Way To Go (highly recommended!)

Robert Rehm, former Dutch Go champion has written a wonderful Go program/tutorial for children/beginners called IMAGO.
MINIMAGO, the little sister of IMAGO is a demo version of this program. It contains the tutorial and the 9x9 playing module. Both programs run on PC compatibles:

minimago.zip
151,784 bytes
Language: Dutch
Author: Robert Rehm

Other programs to get you playing are igowin and igotutor, demo versions of the famous program Many Faces of Go and ez-go. Both programs are suitable for PC compatibles:

igowin
800,000 bytes
Author: David Fotland

igotutor.zip
243,964 bytes
Author: David Fotland

ez-go.zip
149,293 bytes
Author: Bruce Wilcox

If you're a teacher please read Mindy McAdams' How to Teach Go document and Milt Bradley's articles on this topic.