Get a large outdoor garbage bag and use duct tape to tape it shut. Make sure that no air can escape the bag.

Cut little slits into the top of the bag and insert straws part way into the slit and duck tape around the slit so that no air can escape the bag.

One student lays on the bag, not on the straws. Now, other students blow into the bag. If they have to catch their breath have them place their finger over the straw to keep air from escaping.

Watch how the student is lifted off of the ground by air!