Why do teachers ignore it when kids call each other names or pick on others?

For teachers, or anyone who knows, I see time and again kids in school picking on the ‘ugly’ or the ‘poor’ kid with no interference from teachers or staff, particular in 4th and 5th graders.
Is there some sort of teaching that says teachers are not to interfere when kids are interacting this way? These picked on kids come home, sometimes, in tears. What do you say?

My only insight is that the bystander kids have to be empowered to do something and say something. I think most of this goes on in places in school where adults are not around. A group of kids who show they will not tolerate bullying of one of their members is the best defense. It has been shown time and again that if just one person steps up, others will follow. We need to really work on this in the early grades so people do step up and start that chain reaction.