When the teacher attempts to assess who knows what, there are impenetrable barriers. Peer knowledge is very different from the front stage behavior exhibited to teachers as is also the dyadic behavior exhibited by a specially chosen student to mentor. Ryan's research , going way back to the early 70s, showed how unable teachers were to predict who would choose to work with whom.
Students are people.
They are complex. They are many shifting images of their identities, not all of which the teacher has the chance to see. The commuter campus of the millennium is particularly a problem, since our students are more turn by different facets of their identity than ever before. The teacher catches one image. For the teacher trying to fit evaluative reports or letters of recommendation into a crazy quilt of other tasks, that student is peripheral.