In 25 years of teaching I have found that EVERYONE in mixed classes learns more. It does not "slow down" the "Advanced" student, instead it gives them deeper understanding of the subject. "to make knowledge your own you must be able to explain it". I have never had a "special" student who wanted to be part of a group slow the group. All problems I have had with "special" students resulted from them being placed in classes they did't want to be in.