Dear colleague Rose Bard, asked me how I work with mixed ability-groups.
In the last couple of years I have not worked at a school environment. But I can say how I used to deal with that problem.
There will always be strong and weak students in a classroom. And I don't find it a good idea to divide students in to "strong" and "weak" groups.
So what I used to do is divide them into groups or pairs, where there would always be an opposite pair. This way the stronger student could explain what should be done and the weaker student would actually do more work without the excuse that he/she didn't understand the activity.
I also had "be a teacher for a day" game. Pairs of students had to explain a grammar structure, a game or an exercise to the others, that gave them a sense of responsibility.
But that's the easy part.
With my adult classes, though I had a big problem dealing with mixed ability or I should better say "level".
The situation was like that. The school offered intensive courses, promised reaching B1 level in one and a half year. This meant 5 hours a week, non stop testing and going as fast as we could. Unfortunately the school kept accepting students two months after the lessons had started, so I had a 20 people group consisting of people who could communicate, people knowing the basics and people who were just starting. Of course I devoted the time to tutor the beginners on my own time, but still it was not working.
In my opinion you cannot put everything in one basket (as we say here in Greece)
Mixed ability groups can work if you use the right activities. The only thing we should recognize though: there is a different in meaning between "ability" and "language-level"