Successfully teaching to multi-levels (both fitness & skill levels) is the biggest challenge and most important aspect of teaching group exercise. In two words:
Progressions and Options!
Whether I’m teaching a core conditioning class or a choreographed step class it’s the same: I start with a base movement that everyone can do, and progress it with optional variations to make it more challenging and/or interesting. Encourage & emphasize the importance of staying with whatever part of the progression challenges them *enough*.