This is How-to about an atmospheric pressure demonstration.
1. 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. Safety specs and/or screens essential in classroom environment.
2. Place a full double page sheet [preferably from a broadsheet newspaper] over a 30cm ruler placed on the edge of a desk. The ruler should be protruding out about 10-12cm. Hit the end of the ruler sharply with the hand- after getting predictions from the class on the outcome. 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.Beware though!! If the sheet is thick enough the ruler could snap.