I am not sure of the specifics of your assignment, but there is another simple working model you could use instead of the syringe idea. Take 3 empty plastic water bottles. Fill one with sand, one with water, and one with air. The only one that you should be able to compress and decrease the volume of is the one with air. This is because there are spaces between the molecules of a gas unlike the molecules of a solid or a liquid. (Make sure the bottles filled with sand and water are filled right to the top so that there is no air trapped inside.)
It's not a working model, but an operational one, in terms of what I used to do with the phases of matter. I would take a propane tank, small one, a test tube, pair of test tube holder tongs, and a couple of mothballs inside the testtube, along with a plastic cup of room temperature water. I would demonstrate to the class that the testtube with the mothballs contained a solid. The mothballs were in their solid phase. Adding heat energy with the propane tank, I would heat the mothballs up. Gradually, the students begin to see the phase change from solid to liquid due to the presence of heat energy. With this phase change complete, I would continue to add heat energy and then watch as the liquid became into a gas. As the vapor or gas was exiting the testtube, I would wait until there was very little liquid left and then quickly turn the testtube into the cup of room temperature water. The frozen sculpture that results reflects what happens to phase changes when heat energy is taken away. In this model, all three phases of matter are evident.