Resources for self study
Excellent structured course on HTML and CSS, which also addresses networking ideas from Codecademy: includes slides and interactive exercises.
Resources for lectures or group study
Resources for use with pupils
This great Network Simulator allows students to create their own network without giving the School's Network Manager sleepless nights! (Requires Flash)
What is a computer system? Video.
How the Internet Works—A project in Scratch online presentation by Marc Scott (@coding2learn)