What happens if we ask the low achievers to sit in the same group with the high achievers? What is the best idea to handle mixed ability groups?

Ideally, the high achiever will help the low achiever by re-explaining, probably in simpler terms than those used by the teacher, and supporting their classmates. I have actually seen this happen in some of my mixed ability groups,and I often use that combination on purpose. However, I must also admit that it depends on the individual personalities of the students themselves, as some high achievers will be unwilling to help their weaker classmates, and some low achievers will feel too embarrassed to take advantage of the help offered, so that needs to be taken into account.