Sharon Vaughn answers that in the International Reading Association blog site -the following question from a teacher.
What Type of Literature Circle Grouping Works Best—Same Ability or Mixed Ability?
Click here to read what latest research says about ability, mixed, pairing, small group and 1 to 1 instructional grouping. Basic summary use it all!– find your purpose, identify the standards you are addressing, look at your data and provide a lot of high engaged opportunities in the classroom. The wonderful opportunity here is that this recommendation can be done in any language and it will not only support learning of core tier 1 instruction but also provide opportunity for additional options for interventions.