Hi Margie:
There's probably some principle out there that prove a strong student mnatched up with a weaker students somewhow bring the two more closely aligned, to the mutual benefit of both.
I don't know, I've not heard of that, but your description makes for a good case to prove it (at least in that situation.
Student mentoring is cool because it gives students a feeling of self worth and perhaps satisfaction they are contributing. As teachers, we ought to facilitate this kind of thing more, because it's helpful to students.
Regards, Barry