A subscriber wrote to me and painted the picture of her new assignment: a large class of adult students, completely heterogeneous in terms of ability levels and age levels: "What in the world do I do with so many different levels?" she asked.
Adults, of course, can drop out of a class that doesn't meet their needs, so it is a neat trick to give enough to each level so they come back for more. The cost of this, unfortunately is spending many hours of preparing for three or four different groups. Typically, a teacher burns out, teaches to the middle, and the class gets smaller and smaller as the beginners drop out because they are in over their heads and the advanced students drop out from being bored.