Ethnic Diversity in a (Relatively) Homogeneous Space

It's in this context that the other American student I worked with and I were approached by two eighth grade girls after lunch one day, on our fourth visit to the school. One of the girls, D., was a white Danish student. The other, T., was a Danish student of color with Sri Lankan background. D. was more confident in speaking English and approached us to say that one member of their class had been saying, "mean things about black people" and that T. felt hurt by this. D. said that T. was hesitant to talk to the teacher about it and neither of them were sure what to do. We encouraged the students to approach their teacher and said it would be easier than trying to mediate the situation alone. When I left the school that day, I wrote, "I wonder what will happen with this." I also wondered whether I should have done more, been more proactive, or approached their teacher myself. I wasn't sure how much to insert myself in the workings of the classroom and couldn't figure out what my place in this situation was. 

I'm still unsure of how to process this. What is one's place (duty, perhaps?) to intervene in a new or unfamiliar situation? If I were in the position of teacher, how could I have brokered a conversation about the consequences of saying hurtful things? And in a place where there is a much different level of diversity in the U.S. schools that I've attended, how can I avoid either tokenizing or ignoring this particular aspect of identity in the single student of color in the classroom?