In a way he gave a good class in the sense that he probably scared kids out of screwing around with guns.
But he also sent the wrong message that "accidents happen" and even in the hands of professionals guns are dangerous and injure people. That's false. Besides his basic DEA training he'd never taken a class of firearms safety and he'd never done a "train the trainer" course, or taken classes on how to be a firearms safety instructor. He didn't know what he was doing and he was anything but professional.
Going over to someone who knows nothing about firearms and having them glance at the gun and then pronouncing "and see this is a unloaded gun... empty weapon"
Whenever inert firearms can be used for that kind of training they should be. If a firearm with functioning action needs to be used, then it's easy in most cases to take the firing pin out. If someone has to carry while they give a safety class then they should never mix their carry gun / duty gun with the demonstration guns. An instructor souldn't incorporate his carry sidearm into the class.
These inert weapons are fairly cheap! If you're serious about training other people then they are worth the investment.
You can make an argument that as a DEA agent Lee Paige should have been carrying his duty weapon - that's fine. If it would have stayed on his hip that would have been exactly what it was - his duty pistol. But I don't think you can make a case that it was necesary for Lee Paige to bring a loaded gun to a training class as a prop. When he unholstered his duty sidearm - that's what it became: a loaded handgun used as a prop in a training class.