I think the important part of the story is both white and black students left the ceremony but the white students were allowed to leave and ONLY the black students were made to move to the back of the bus. The white kids who did the same behavior were not punished in any way.

I think people are going to skip reading the article and do some "what's the big deal, can black people NEVER sit at the back of the bus now? snowflakes are too sensitive" but it's specifically that the chaperone singled out only black kids from a group and only punished them. In a way that happened to also mirror history. The fact it was a racially charged punishment is more a cherry on top, with the "white kids can leave, we will only punish the black ones" being the primary outrage.