If you aren't familiar with it, you should be - it's easy to do in any setting.
Fill a cup part way with water. Place an index card on top (make sure the card is large enough to completely cover the cup).
Pick up the cup, placing one hand on top of the index card.
Turn the cup over, holding the index card up with your hand.
Ask your students what they think will happen if you remove your hand that's holding the index card.
Then do it!
There really is water in the cup, I promise! My hand got in the way of the water line...
Air pressure doesn't just push down, it pushes on things in all directions. In this case, air molecules are pushing on the index card. They exert more pressure than the water pushing down does, so the card stays up.
If something breaks the seal, or the index card gets too wet (as it will after enough time has passed), the card will fall and water will gush out!