I do not think the kids at the 99.999 percentile as referred to by @roycroftmom benefit much from having classes with those at the 90-95 percentile, nor should they need to learn how to explain so that others can understand as @twoinanddone mentions. This is similar to elementary school having the kids good in math teach the kids that are struggling. It is fine to do occasionally but not regularly.
Ideally, these top kids could be challenged so that they can advance which in turn should benefit everyone.