Google Sketchup is a powerful free 3D modeling tool from Google which enables anyone to feel like an architect. I have used it to teach my students about 3D geometric shapes as well as to get students to model the school. The program is easy to use for beginners who want to create simple 3D designs and powerful enough to allow more advanced users to create detailed architectural drawings.
If you have not used this free software before, I strongly suggest you get it and test it out yourself. My grade 4 students love it and are able to create reasonably detailed models in a short period of time.
If you are ambitious, you can get students to model your school and have it added to Google Earth for others to see. This is probably not something that is easy for an elementary school class but certainly possible for middle of high school students.
Google: Sketchup CD Case from Transvideo Studios on Vimeo.