How about rolling out tiles and have them work in groups to incise
designs or pictures into the clay? Tell them that the next class will finish
the glazing for them- as a surprise. It could also give you something to have
for those kids in the next group who always finish a project early.
Or do you have any old pots that could be broken to use as mosaic
pieces? Again having them work in groups?
Good luck! Filling in those last few days is always tough.
Monica