Yes these are assumptions and yes there's always exceptions. For example, this could be a publicity stunt, maybe he was taking the piss. But this case here is not the same as making generalizations about groups of people based on looks. It's not the same as thinking that because someone is dark skinned and has a beard they're a terrorist. There are different kinds of conclusions one can relatively safely reach based on outside appearance. For example, if you see someone with cauliflower ears you can safely assume that they engage in some sort of wrestling, jiu jitsu martial arts sort of thing and conversely it might be a good idea not to initiate a physical confrontation.

My point is, drawing conclusions and forming a rough profile of someone based on looks is a natural process and one should not shut it down. Just keep an open mind at all times.

This particular guy in the picture is not profiled based on gender, nationality etc. It's the context of the class subject and the very particular attire that make people jittery. Not saying don't give him the benefit of a doubt, but I do think you can pretty safely assume that he is probably not the most responsible person to give a lethal weapon to. That's all.