Creating a model lung is pretty simple. You can find directions all over the internet, including right here!
Start with a plastic bottle, any size will do. (Pictured here is a water bottle, but 2 liter bottles work as well).
Cut the bottom off the bottle. If you're having students make their own lung, you may want to do this for them, and if the students are young, you may want to tape over the cut edge so no one gets cut.
Place a balloon in the neck of the bottle, and stretch the opening of the balloon over the opening of the bottle (see the blue balloon in the above photo).
Cut the narrow part off of a second balloon. Stretch the remaining balloon over the bottom of the bottle.
Now to use it....
The blue balloon represents a lung. The red balloon is the diaphragm.
When you breathe in, the diaphragm contracts (pull the diaphragm balloon down). This lowers the air pressure in the chest cavity (because there's more room) and air fills the lungs.
When you exhale, the diaphragm relaxes (release the balloon, you can even push up on it a little). The air pressure in the chest cavity increases and air flows out of the lungs.