Upon being a creeper and looking at randoms' photos of their last year at highschool - I now get it. A) Their grade was tiny, our was flippin huge, I sure didn't know half the people in it. And B) they had free dress days, and gender swap days, etc. They were allowed to have fun! It's an important step towards comraderie among a grade, those fun and free days.
It makes sense, when I compare high school to my primary school days. Those were the differences.