You also should be concerned about the social dynamic that exists for students quite separate from your class. If you ask students to peer review (in essence to judge one another's performance) how will this be affected by outside social factors? No one wants to be a tattle tail or teacher's pet, or just plain unlikable. Setting aside the validity of the labels I believe we can recognize that they exist. While grades can motivate them to drop the hammer on a bad worker, the effect on the social dynamic must be considered. Depending on the stage of development it can be very important to promote a harmony among the student body and while actions that promote enlightened discussion are good, actions that may lead to grudges or ostracizing are not. Too often I believe teachers try to take a moral high road in ignoring the existence of a complex social hierarchy. Teachers actions should promote the dissolution of such separations. As much as possible teachers must avoid actions that could easily create ill will between classmates.