I would think they also make dummy plastic cartridges, for use in demonstrations of loading/unloading techniques. There was certainly no reason to have live rounds in the gun, for any demonstration that was going to be happening in the classroom.
Those are commonly called "snap-caps" and they're not for loading/unloading.
They're designed for "dry-firing", i.e., practicing a draw from holster/point at predetermined spot/pull trigger.
They're designed to provide the proper resistance for the firing pin to keep from overstressing the firing pin spring.
They do make dummy rounds but this was caused by another kind of dummy.