Safe explosion demonstrations
From our friends at Periodic Videos, the following come to mind:

Gun cotton, more precisely known as nitrocellulose, which is a very simple nitration that you could show prior to the pyrotechnics.

Sodium and Potassium are also good, though you may not always get an explosion, depending upon the available hydrogen above the metal.  Which suggests...

Hydrogen! This is the classic explosion in oxygen. In fact, on a small scale, the electrolysis of water into hydrogen and oxygen can be used to show that the mixture of these gases is very explosive. But keep it small!

All of these experiments avoid outright explosives, which seems prudent in a high school setting.