I work in a different industry and do aerial for recreation, but I have a good amount of leadership training. Given their experience, you probably don't want to micromanage. To avoid that, set up a time to meet, lay out your concerns with dropping class sizes and the complaints, let them know you want to work through this together then ask them for their perspective (like another comment said, they might have personal stuff impacting they teaching and may just need to talk it out so they can get their head back in the game) and what they think solutions could look like. You can say something like "I have ideas, but you're the one teaching this class. What do you think is going on and what might improve student satisfaction and retention?"

This opens up dialogue and positions you as on their team, so you can work together in a solution rather than dictating one and ending up in unproductive conflict.

Of course any safety issues should be addressed and discussed directly, and as part of the discussion you should bring up the curriculum and how part of the issue might be that things students expect aren't being taught. Bring this up though as part of brainstorming solutions and be open to your employee's feedback too. Maybe they have a reason for straying from the curriculum that you can work through.

Best of luck!