craftgrrl | air dry/self dry clay

Hey crafters,

I know some of you on here use clay and was wondering if anyone could recommend a good brand of air dry/self dry clay. I have 25 students (ages 5-11, woah!) next month, for only one week, 2 hours per day. I want to do some sort of sculpture project but don't have a lot of time to spend on the clay drying. If possible, I'd like something that would dry overnight. Also, something that can be painted with acrylics or tempera.

Thanks.

p.s. I may have posted here before about this, but the art camp theme is "Down on the farm" so any little art projects anyone can think of that a 5-11 year old can do, that is farm themed please let me know. The above clay project will be about making their own weather vane ornament. Somewhat farm themed.