Robert Marzano, Debra Pickering, and Jane Pollock, in their book work, Classroom Instruction That Works, explain that different groupings have very different outcomes depending on the student.
What is best for the middle of the road student?
For the student who is achieving right in the middle, homogeneous grouping gets dramatically better results. As you have probably noticed, the higher achieving students tend to take leadership roles (or control) of groups when you have mixed abilities. When all of the students in the middle are working together, they have more chances to take on leadership roles and actually end up performing at a higher level.
What is best for low-achieving students?Heterogeneous grouping works much better for lower achieving students. The group tends to pull up these students. The students get exposed to higher rigor and content with their peers at different skill levels.
What is best for high-achieving students?If you ask your high-achievers, many of them will say that they just wish they could work along or by themselves. Most of them resent constantly acting as the tutor. Heterogeneous groupings do not increase outcomes for these students.
So, what should you do?
Well, really that is up to you! But here are some ideas . . .
You might want to use a combination of different grouping styles. Knowing that for 80% of your class, homogeneous groupings gets the best results, and for 20% of your class heterogeneous groupings is the way to go, you probably want to use a mixture. Don't do all heterogeneous groupings. At the same time, don't to all homogeneous groupings.
You might want to combine up cooperative learning with small group direct instruction. One thing I do that has been successful is give my students a mini-quiz (1-2 items) before I assign groups. I then place my top 80% in homogeneous cooperative learning groups.
I do not, though, put my lowest 20% in a cooperative learning group. I instead spend time teaching strategies and reteaching the specific skills that these students are missing. I sometimes let them know that they can join a group when they meet mastery of the skills. This only works, though, if the rest of the cooperative groups really know how to work together and can be fairly independent.