Top set groups will benefit from having pupils that can support and motivate each other, leading to accelerated learning.
Peer coaching/support could become embedded in every lesson.
Bottom set groups will be able to do tasks that focus on lower level social skills together, without more skilled pupils not being stretched.
Focusing on social skills in that way could result in those pupils making significant progress in a short time period.
Bottom set pupils would lack role models modelling higher order social skills.
Bottom set pupils would also suffer from not having higher skilled pupils supporting their own learning.
Too many leaders in a group could make it difficult to allow them all to take on roles appropriate to their level.
This would allow pupils of all levels to take on a role suited to them. The more highly skilled pupils would act as role models, and support the learning of the less socially skilled. However, all lessons would be vulnerable to potential disruption, pending on the number of pupils at lower end of the social skills ladder.