Why not try making a Roman mosaic? You can make a simple mosaic just with black and white pebbles, which you can get at any gardening store (or at the beach maybe). You could set the pebbles in plaster of paris, or in cement the way the Romans did it.
Or, in a classroom or at home, you might want to just cut out lots of little squares of colored paper, and assemble them into mosaics with glue. For the best results, make sure to have some light red and some darker red, and some light blue and some darker blue, and so on, so that you can do some shading. The pieces should be about 1 /4 inch (1/2 cm) square. Look at a lot of pictures of mosaics first so you can see how the Romans did it.