madeinuk, you have made a valid point about the relative merits of peer-tutoring.
While there is good and bad in everything, if peer-tutoring was found to be such a strong classroom strategy for reinforcing learning, possibly the teachers would have the lower performing students engage in teaching each other, while the students who've demonstrated readiness/ability to move on to other material, do so. This approach may more closely mirror the "aha!" moment described upthread when a person struggling with a concept talked it through (to Dude) and in doing so, came to a fresh realization and solved their own problem.
That said, acceleration, including single subject acceleration, even into a mixed-abilities (heterogeneous) classroom, can be quite effective both academically/intellectually and socially, expanding the child's circle of friends. There are many threads on the forum about acceleration (single subject acceleration, whole grade acceleration, multiple grade (radical) acceleration). The Iowa Acceleration Scale (
IAS ) is a tool often used to document and weigh pros and cons of acceleration for a particular child and learning environment at a given point in time.