Those classmates a disgustingly rude, but sadly this isn't really an uncommon scene. Somehow when a person looks unconfident and meek people feel freer to criticize them, in fact it attracts critical people like sharks to blood. You see it in groups of people all the time be it school or the workplace, there is often someone everyone feels entitled to be "above" or "better than." It's not even always with malicious intent but just the imperfect nature of people wanting to compare and be superior and something that we have to consciously resist.
On the flip side, its never a good idea to pin your confidence/happiness of the opinions of others. No matter how beautiful you are, there will always be someone to criticize you and want to drag you down. We can see this clearly with Jeewon's character. Not that it's easy to completely ignore the opinions of others, but it's a filtering process. You have to decide whose opinion counts for you.