For what it's worth,
 

I think explosive devices are only used when necessary.

"Demolition Man" with Stallone and Snipes used a higher-tech version of what I understand as the "pin explosion", i.e. a powered "pin" used to shatter glass.

The scene was where Snipes was firing some weapon at Stallone as Stallone ran through a room full of display cases. The large, glass panels were shattered using some sort of relay that activated a small, short metal rod that acted as a hammer of sorts. Each "hammer" was located near the bottom of each pane of glass. For the shot, the relays were actually hand-activated by someone off-screen and timed to Stallone's movements as he passed each display case. In that instance, I believe they used real glass. (You can learn a lot by watching HBO for "The Making of...", and "Mythbusters" also presents some creative ideas.

My suggestion of a hammer and nail from underneath is a cheapo version of the pin explosion...without the explosives or electric relays. A spring-loaded pin would probably work as well.

Whatever methond you use, just be sure to have a few extra glasses handy for multiple takes.

Let us know what method you end up using.

Thanks for all the replies, guys. The shoot isn\'t for another few weeks, so I have lots of time to prepare.

It looks like I might be going the route of the BB gun, simply because it is the only piece of equipment mentioned here that I can get for free. However, I do plan on trying out Denis\' technique of the hammer and pin - I\'ll do a few tests and see how it looks.

Hopefully it will work, as safety is a huge concern, especially in a public school environment.

So thanks again to everybody, you\'ve proven once again why I come to the DVi in the first place ;D

~Loggie B