My approach was simply to first, acknowledge the reality of the situation and then, based on the results of each student's entry test , prepare a curriculum for the levels represented and devote an equitable "time share" for each level. After some initial adjustments for both the students and me, it became rather routine. Of course, the number of students is another consideration. In that environment I usually had an average of 10-12 students. I'm sorry that I don't have an easy solution for you.