I also tested a number of air dry clays this summer. Many do not feel much like real clay. All of them are not very strong, but they are a bit stronger than ordinary clay.
Of all that I tested, Amaco Stonex has the nicest plasticity and general workability of anything I have checked.
The self-hardening clay from Sheffield Pottery, Inc. in MA. is very dark in color, dries nearly black where the soluble ingredients come to the surfacer while drying. It is half again as strong as the dry clay from Amaco, but the Amaco has considerably better working characteristics. East coast schools could order from Sheffield for less shipping cost than from Indianapolis.
At this point, unless durability is seen as a major problem, I would stick with the Amaco Stonex. Amaco also has one called Marblex, but it seems very similar to Stonex.
On the other hand, if strength and durability is a problem, a clay called Della Robbia seems strongest of what I have tried so far. I seems reasonable to work with and it can be be oven baked for added strength. It is nearly as nice as the Amaco Stonex to work with. Without baking it is about twice as strong as the Amaco. After the Della Robbia is baked at 250-F in an oven it is about three times as strong as the Amaco. Hence, oven baking could be optional. It is brown, so not quite as nice for painting as the white Amaco Stonex, but the brown looks like natural clay.
There may be others that I did not test.
Sculpty is not clay, but is an oven fired plastic. Oven fired plastics are more expensive than clay, and if they over fired in the oven they may produce toxins, and may leave toxic residue in the oven. If you use it, be careful not to over fire it. After using the oven for firing plastics (and maybe even oven fired clay), it may be best to do an oven cleaning cycle with the vent fan running prior to using the oven for food.