Unfortunately, our knowledge about "peer assistant" is merely a "knowledge".
In fact, most teachers do not notice the power of peer assistant concept for their students, especially when they are teaching teenagers.
As we know, teenagers have closer "bound" with their mates...well, at that age, we understand it, right?
PA, creates more comfort for them to learn, cause they can "stay" with their mates, instead of their teacher in "formal" manner. Of course, as teachers, we have to make sure that the comfort won't lead them to "chat" instead of "study". However, this will make them learn more about commitment and responsibility (believe me, I've tried it. It takes time, but worth it)
PA team gives more chance to the member of the group to learn the materials more intensively. And if the team has any problem, the teacher also has chance to give guidance more effectively, cause she simply talk to smaller groups.
Of course, as always, teachers should be able to decide, what kind of materials are suitable using this concept. Besides that, make sure that before starting the concept, we give enough motivation to the students who are good at our class/subject, so that they would be happy to join any PA team and be the "helper" for the other students (especially for those who belong to the slow-learners groups). I myself, call them as my assistants. And I always say to them that a teacher is getting improved in skill and knowledge when she should face/treat "difficult students", so they do not need to feel pity to share and help others. That is not only "good for their character building--as a person", but will also good for their mastery and competencies. So far....it's worked. :-)