The Plunger can be found in the girl's bathroom in the Toolbox. It can be used to unclog the toilet on the far left making that toilet usable again.
You can't break anything with the plunger, only push around the Popcorn Bag, Popcorn, Soda Cans, Crowbar, Toolbox, Cardboard Box, and the Flashlight.
The plunger is also pretty deadly as hitting a friend once and knocking them back will injure them, and a second will kill them.