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The answer for the question will be too long to put here. Anyhow there are many techniques followed in such situations, along with various assessments that should be conducted for the students involved in the process prior to starting the teaching process - that would bring about good results.
From my own persona; experience I can say that this is NOT good to mix a group of younger students with older ones - their attitude to education, discipline, mannerisms, care, attention and all are totally different. This leads to awkward moments and points of certain tension. So, I would never allow such a group to function and would immediatelly send older ones to another group
I agree with experts answer, Thanks for the invitation.