Clothes can be like a mental shortcut to ascertain what someone's role is, what they might be focused on etc. For example, if you see a white guy in baggy clothes and big dreadlocks, the mental shortcut would probably be that he smokes weed and he isn't a workaholic. You could be wrong of course, but most people probably don't give much thought beyond that initial impression, at least until they talk to the guy and see if their expectations are confirmed. The same applies to wearing 'smart' clothes in the opposite direction. It is silly in a way that we have defined 'this is smart', 'this is not smart', there is no reason for it apart from that society has made these arbitrary rules. It just shows people that you are in the mindset of doing the tasks that people wearing these clothes usually do (office work, meetings). There is also an argument that dress clothes are more stylish, but this doesn't really apply to high school of course.