Hello- I'm making a grad cap cake topper for a cake on Saturday. I'm covering a styrofoam ball and a piece of cardboard. Does anyone know if it will be dried out and crumbly by Saturday? How should I store it?

It should be fine.  If you use gumpaste, 50/50 gumpaste & fondant or fondant w/tylose and you make it now, it will be nice an dry by Sat and you won't need the cardboard.  

I just made two over the last two weekends w/ a 50/50 mix of gumpaste and fondant and I just left them on a sponge board to dry all week.  they were perfect by the end of the week.  But, you better make it today if you want it to dry on time.