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Platform Gamemaker Lesson Plan
This is a great, easy introduction to Gamemaker and object-oriented programming.
You'll need to install Gamemaker and download the zipped files, which include the lesson plan. In the class, kids build a new level, or room, in an existing game.
I've done this class with kids as young as 7 or 8, and it's always been fun.
Platform Game - Word Document
PlatformFiles - Zipped files needed for class, including the pdf file from the Gamemaker website that shows how to build the entire game from scratch; if you wanted to do that much more complicated class, you'd need to go to the site and download all those files.