I'm amazed at what adults don't see (or perhaps don't want to see).
I was at a park with my toddler the other day and a school group was there. I spent a lot of time watching one girl who I eventually realized was friendless. She did a good job of hiding it -- waited around for a swing; swung for a while; loitered casually near a group to look like she was involved -- but she was clearly stressed out and working very hard to not look alone. What was most interesting of all was the teachers, whose only interactions consisted of joking with the popular kids. The whole situation was clearly torture for this one kid, but all the teachers could see was that the group was having fun.
I think a lot of these people are emotionally still in 6th grade themselves -- wanting to be liked by the popular kids, not wanting to have too much to do with the unpopular ones. I even wonder if they are deliberately blind to some of the bad behavior of the popular kids, because they don't want to lose favor with the group. Much easier to discipline the kid that nobody likes.