I agree with everything that Kriston says, especially in the regular "traditional" classroom, where a student teaching is not the norm.
However, I've seen positive examples in Montessori, multi-age classrooms. Most Montessori classrooms have children of up to a three-year span grouped together. As the students get older, they are expected to lead the younger students in some activities. This may coincide with the older/advanced student learning another aspect of the activity from the teacher. In this way, they are formally reviewing (when they teach the younger child) and learning something new.
Because it happens in addition to learning new material (including leadership skills) it doesn't seem to hinder growth at all. Certified Montessori teachers are also trained in observing student needs and differentiating work for each student. Because the classroom contains children of multiple age groups, differences from the age/grade lock-step are not as obvious.
