This essay explains why some educational policy-makers and researchers are opposed to ability grouping. The comments at the site by Sherrie273, Django, and Brianna are much more persuasive to me.

http://blogs.edweek.org/teachers/living-in-dialogue/2013/05/michelle_newsum_tracking_our_w.html
Michelle Newsum: Tracking Our Way to Wider Achievement Gaps
Education Week
May 29, 2013

Quote
The National Association of School Psychologists (NASP) supports the instruction of students within heterogeneous classrooms that recognize and accommodate individual student differences in learning style, ability, and interests. NASP opposes the use of tracking because of its demonstrated negative effect for many students.

Research has demonstrated that the use of whole class ability grouping disproportionately impacts minority students, economically disadvantaged students, and students with lower ability. Related to individuals identified with educational disabilities, whole class ability grouping does not comply with the requirements of placement within the least restrictive educational (LRE) environment. Further, the practice of whole class ability grouping/tracking can deny many children of their statutory right to equal educational opportunity. Demonstrated best educational practice can lead to the establishment of excellence for all learners without resorting to the use of ability grouping. Such positive educational practices supported in the research and literature include:
Cooperative learning, differentiated instruction, small group instruction, curriculum modifications, scaffolding, essential understandings, structure of disciplines, learning communities and flexible grouping.