[jtbigwoo]

It turns out to be more complicated than you're thinking. Lower- and average-performing kids get a greater benefit from mixed classes than higher performers. However, high performers with some interaction with average performers do better than high performers with no interaction outside their ability group.

[alistairSH]

Does that interaction have to occur in a highly technical subject like mathematics? Or, does mixing achievement in PE and elective courses have the same impact?

[jtbigwoo]

It's not clear from the articles I've read. Here's one that references some of the research:

http://educationnext.org/all-together-now/