Lollipop ghosts are a very easy, kid-friendly craft and great for those school parties. Pick up a bag of tootsie pops, a box of facial tissue (or you could use tissue paper), and some Halloween ribbon. Make sure to have a black marker handy. Put one tissue over the top of the tootsie pop and tie at the bottom with the ribbon. Use the marker to dot on some eyes. Simple, yet fun!
You could also adapt this idea to hang outside in a tree or from your roof. Use crumpled newspaper and white trash bags with ribbon to make a floating ghost. Secure with fishing line so it appears that they are floating.