Porcelain is viable if you want something unglazed, but otherwise I
would stay away from it. In order to reach maturity, it needs to be
fired to a high temperature and, unless you use a very elaborate set
up, you will only be able to glaze one side. It can be colored by
mixing Mason stains into the unfired clay, but Mason stains are
expensive, as well as a little tricky, especially if you want
saturated color rather than pastels. On the other hand, an unglazed
white porcelain ornament can be quite lovely--I used to know someone
who made these. (If you decide to go the unglazed porcelain route,
email me off list and I'll describe the tricks I used for easy and
relatively inexpensive firing.)
If you want to get volunteers involved, check out the kind of low
fire white clay used in ceramic "painting" studios. It won't be as
strong, but it will be cheaper in several ways.
The casting process in ceramics is quite different from the jewelry
process but, if you are doing something with a one-part mold, the
learning curve is pretty short. Does the art center you're doing this
for have a ceramics studio? If so, talk to the ceramics teacher, who
will know a whole lot more than I do!
Lisa, Maybe you can answer a question about porcelain for me. I have
a porcelain pendant that I want to make a mold of with a vulcanizer,
can the porcelain withstand the pressure of molding it this way?