I've just started doing castings of students' hands using dental alginate to make the mold, and then filling the molds with plaster. These are such a hoot!
One of my seniors did this in the fall, but now it's a project that my advanced art students are doing as part of a unit on sculpture. At this point, based on the cost of the material, I'm starting with the seniors, but if I have enough of the alginate I'll let other advanced students do it, too. This is not cheap....
I learned about this project last summer when I was teaching at a summer art camp. One of the teachers was doing this with her middle school students, and I was fascinated by the quality and detail--and knew I'd have to try it. :-)