There are several ways of dividing a class of children into smaller groups. It is fun, but time consuming, to have them arrange themselves in alphabetical or age order. If you believe in star signs, and opt for the age thing, you may wish to mix them up a bit afterwards to avoid having all the leading Leos in one group and the recalcitrant Capricorns in another.
My own favourite, because wherever possible I begin work in a circle, is to give each child a number. If I want five groups, the person on my left is number 1, next to them is number 2 and so on round. The child next to number 5 is number 1 again. Once numbered, all the ones gather together in their group, as do the twos, threes and so on. I like this because it's fast, transparently arbitrary, breaks up subversive cliques and mixes boys with girls.
The other day I found myself working with eleven Year 5 children (aged 9/10). It was my first time in the school and we had 45 minutes to produce some music for a performance based on Creation myths. They nearly all had formal instrumental lessons and most were obviously very bright. Being young enough to 'play' they took to improvising very well. I would give each sub-group a title and they would play a convincing 'Sprinkling Stars' or 'Building a Mountain'.
Twice I split them into groups, using the method described above. The third time, when we chose groups for the imminent performance, they asked if they could choose their own groups. I was dubious about this but they had worked very well so, after a brief discussion about the potential pitfalls, I let them get on with it. This gave me a strong group of three and an able and reasonably cohesive group of four. It also left four less able and apparently low status children who were not a group. None of the others wanted to work with them and they didn't especially want to work with each other. Fortunately this was a selective school (albeit on religious grounds) in a well-to-do area and the 'less able' students would be shining beacons in many other establishments. The four remaining children accepted the result of the process to which they had agreed and coalesced into a functioning group. Each group produced wonderful music in the final showing to their peers and younger children, much to my relief.
Most of the schools I work in tend to be in less favoured areas and the proportion of socially able, high achieving children is lower. I am not in a hurry to repeat the 'sort yourself into groups' experiment in any of them. The fact is that able children, and I mean able both academically and socially, want to work with each other. They are usually capable of cooperating in ways less able children simply can't. But they are also capable of leading and inspiring the less able and can derive benefit from this. If there is a small amount of dumbing down from their perspective it is greatly outweighed by the overall rise in quality. And ultimately, in music at least, the session has to work for the whole class for it to work at all.