These are just a few of the game design tools I've been researching in the past few years. It is possible to integrate any of these tools within your existing curriculum. The goal is get student to transition from playing games to creating games.
Gamestar Mechanic
This is a web-based game design tool. It very easy to get started with the tool, and I would recommend it for Elementary and Middle School students.
Scratch
This tool is great for Elementary and Middle school students.
GameSalad
This is a great tool to create games for iPhone, iPad, and Android devices.
Alice
I would recommend using ALICE withi High School and Intro Level College Courses.
Microsoft Kodu
With KODU you can design games using your PC or with an XBOX. I would recommend this tool for Upper Middle school and High School classes