This is How-to about an atmospheric pressure demonstration.

Safety

For the 1st demonstration, safety goggles and/or screen(s) are appropriate to protect againt 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. Also, don't lean on, or sit on the ruler: it'll make for an impressive demonstration of the powr of atmpospheric pressure and may make you feel better for getting away with some cool and reckless experiment that YOUR professors would've chewed you out for while you were secretly formenting your jealousy that they could and probably would get away with it and plotting your revenge for, which has finally come to hand, after all these years at normal school, it could break unexpectedly and send you for a nasty fall.

Nature abhors a vacuum

Briskly boil a small amount of water for a short time in a cola can (until it's 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.

Pressure per square inch/meter

Place a full double page sheet (preferably from a broadsheet newspaper) over a ruler or yardstick 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.