School Science/Atmospheric pressure demonstration
For the 1st demonstration, safety goggles and/or screen(s) are appropriate to protect against aluminum shrapnel or other risks. For the 2nd, be careful that the setup is not near any strong heat sources or objects that could be easily upset as the paper could go flying.
Nature abhors a vacuum
Briskly boil a small amount of water for a short time in a soft drink can (until it is full of steam) Then, using a pair of tongs (or clamp if your tongs are too small), swiftly invert the can and plunge it into a large beaker (bowl or bucket) of cold water. The steam condenses, the pressure inside the can falls, and the can collapses (is crushed by the higher atmospheric pressure weighing on the outside of the can). The can must seal on the bottom of the dish with the water, otherwise the vacuum created in the can will only draw water up into the can, and the can will not collapse.
Force divided by area
Place a full double page sheet (preferably from a broadsheet newspaper) over a ruler or long stick placed on the edge of a desk. The ruler should be protruding out over the edge. Get predictions from the class on the outcome, and then hit the end of the ruler sharply. If done well the page will stay put, or rip. The air pressure pressing down on the newspaper holds the newspaper down despite the force of the ruler trying to push it up.Really you do not need air pressure to explain it, the air pressure can be the same above and below the paper. To show that is not the air pressure you can use two more rulers at both sides of the protruding ruler. Now the air pressure is the same above and below the paper, and the ruler broke again. It is the mass of the paper that broke the ruler. Using a small piece of plasticine on the ruler with the same mass as the paper, you may use the ruler again.