Roger: Among year 2 medical students, my opinion is that no criteria are needed to balance academic achievement, since everyone in the class has made it into med school and through year 1, demonstrating competence above minimum acceptable level. When teams consist of 5-6 MS2s assigned by a randomization method, it's VERY unlikely that any single team will be handicapped or overly endowed with learning capacity. The only sorting criterion we use in second year is gender; we don't allow the random sort to produce all-male or all-female teams; if that does occur, we move a few students before the first TBL assignment to insure that every team has at least one male and one female.
You will find the attached article on "collective intelligence" relevant, I hope. /Paul