The volume of air in sand can be calculated by adding 40 ml of water to 40 ml of dry sand. The water will fill the air spaces in the sand and the top surface will be at about 65 ml. Since it would have been 80 ml without the air, the air must take up 15 ml of the sand. By dividing 15 ml of air by the 40 ml of dry sand, the percentage of air in sand is found. To ensure safety of your graduated cylinders, make sure the rocks are too big to fit in them.
Plastic graduates are always a good idea in the junior high classroom.
Use length x width x height measurements to find the volume of the wood block. Use the graduated cylinder and water displacement for the marble. Try the same measurement of the marble using the beaker instead of the graduate. It doesn't matter how much water you start with, just leave room for it to rise. Use the overflow cup, the graduate and water displacement for the rock.