From the Google for Educators site, have a look at the SketchUp for K12 Education page. SketchUp is of course a free resource, but if you are an Educator, you can also take advantage of a free copy of SketchUp Pro 5.
In addition, if you are looking for printed SketchUp resource material you can order the SketchUp 5 Student Workbook in either hardcopy or PDF format. For even more, there is also a SketchUp 5 Instructor Guide. I just downloaded a free evaluation copy of the workbook (PDF) and will spend the next few days going through it.