What suggestions do you have for making classes useful for multiple levels? 

I have had success teaching many multi-level classes. Think of a classic yoga class. Most yoga teachers provide modifications as well as an option for going deeper. Teach your class towards the mid level of the group. Provide one option for modification and one option for going deeper. For example, consider teaching a basic movement. Offer a focus for technique for mid level students, but remind newer dancers that they should just focus on getting the rhythm and shape during the lesson. Finally, give a styling option for more advanced students in the class. This gives all three levels something to try during the class and they will benefit from dancing with a mixed level group during the class. You might even use the terms “level 1.” “level 2,” and “level 3” while encouraging people to focus on whatever they feel most comfortable.