Personally, to me it struck me as something I would try on a group of first graders (provided I knew I wouldn't be sued) but not on a group of adults. They know it's just a game, and treat it as such, but nobody's going to refuse because to me that sounds like a very fun game (approached from the right mindset, anyways, and provided you make sure the audience doesn't take it too far. I'd probably hand pick people to "criticize" and make myself a member of that group so I could step in if another was being problematic). So they all do it, and they all think it doesn't matter outside the game, except that since it was so unusual a thing to ask them to do they're going to remember when you tell them why they did it at the end of the lesson, and they're not going to forget it anytime soon. Preferably do it when they would normally be having a "normal" lesson.

Harry's army is about four years too old for that angle to work, so I wouldn't expect much of the "training". I'd expect more from the entirely conscious chain of reasoning that they respect General Potter, and that he has them do all kinds of weird "training" things and an awful lot of them turn out to be good ideas, and that he told you outright that he thinks this is an important thing for you to try to do. But then, that's a conscious chain of thought, and by the time an issue like this hits conscious thought it's already passed all the lines of defense that matter. So I wouldn't expect much of it. But hey, he's already employing a scatter shot approach towards their weirdness training so if this one idea doesn't work out it doesn't cost him much more than the time it took him to plan the exercise.