I currently co-teach a Biology class in which we use cooperative learning (lab) groups at least once per week. We have various methods of grouping students (groups are typically 3-4 students):
-heterogeneously grouped lab groups (teacher chosen)
-randomly chosen groups:
- students pick names from a jar
- students count off 1-4
- students match puzzle pieces
-homogeneous/heterogeneous groups/partners chosen by students (we find that typically students get a lot less accomplished when they choose their own groups or partner!)