You asked specifically which outfit of those suggested was the most Scottish and you got an excellent answer from Jock Scot. I certainly agree with him about the flat cap.

To the more general question (What is most Scottish?) I'd have to agree that statistically, the "jeans, jumper and trainers" answer is probably the right answer.

But it occurs to this Scot, living in England not the USA, that I don't fully understand the purpose of "Cultural Day" or "Cultural Night" in American Schools. I assume it is a chance to look at how life is different in other parts of the world. If so, just trying to present a statistically accurate portrayal of Scottish dress today - or even just presenting daytime dress - might be too limiting. If the idea is to illustrate different aspects of life in Scotland today then you might just as well wear what I, a Scot, would wear to a Burns' Night celebration this year i.e. PC, black tie, kilt, hose etc.. That is just as typically Scottish. But then so is someone wearing a Rangers or Celtic football strip to go to a match