US Supreme Court on Peer Grading

I'm always surprised by what I learn or see from reading Supreme Court rulings.  This time, I find an extremely succinct explanation of the positive impact of having students mark each others' test papers:

Correcting a classmate’s work can be as much a part of the assignment as taking the test itself. It is a way to teach material again in a new context, and it helps show students how to assist and respect fellow pupils. By explaining the answers to the class as the students correct the papers, the teacher not only reinforces the lesson but also discovers whether the students have understood the material and are ready to move on.
— Owasso v. Falvo