Hyper-tufa is my favorite summertime sculpture technique. There are several recipes and it seems everyone has their favorite.
I use equal parts portland cement, sphagnum peat moss, and Sand, vermiculite or perlite. I've included some links from my favorite Garden Junk site. :-) The neat thing about this medium is that it is a great for outdoor artwork. It is deceptively light. I don't know if I would do it with students because of the cement - should I continue this practice or allow students to do this with some precautions?