Various "stunts" to attract crowds (and hopefully students) have been around since day one of the martial arts.
Probbaly started when someone asked Og the caveman to "show me what you got " with his club.
Og thinks "well I can't just kill this silly jerk, his folks have the bear jerky consession--and I LOVE bear jerky...what to do...what to do...
So Og goes and smashs his club thu a big, thick dead branch-- to show the power of his swing and the strength of his clubbing technique.
All kidding aside, short of actually harming people, its hard for me to think of a "clear" demo--too many potential variables at work.
For my money a good demo would give the watchers a clear picture of what training is going to be like in that school---as well as what you might be able to do with it.