I've come across an application that seems to provide the ability to draw, design, and modify drawings in 3D and to do so with, if not absolute simplicity, at least with a relative lack of complexity which makes it seem great for students to use, even at Middle School age.
The program is called SketchUp. To quote from their introductory blurb:
"Developed for the conceptual stages of design, this powerful, easy-to-learn software allows for quick and easy 3D form creation, modification and communication. Sophisticated enough for complex projects yet accessible for beginners......"
This may be something "down the road" in terms of use at the Middle School, but SketchUp seems to provide a great deal of power and "user-friendliness". In other words, this is an application to keep in mind for a long time.