For many things, if the student tends to lose attention or dawdle, you need to do some attention training. Set the lesson time very short… say 5 minutes. Let them learn to keep attention for that length then slowly increase it. See the learning to keep attention as the lesson, not the subject itself for a while.
If you just can’t see yourself doing that…. then….
For readings, watch the student and stop reading just BEFORE they lose attention. That takes observation and practice on your part.
For ‘seat work’ I agree with above. Set a timer, make sure the amount of work could be done with a bit of room. If they are not done, go onto the next subject, and they can do it during ‘their’ time.
Or
If they aren’t willing to do the work of learning, they better get used to manual labour – so set them to work cleaning… and not easy stuff either.
I have a Suzuki story….. There was a student in violin that was having problems working on vibrato and was brought to Suzuki. Suzuki asked him to play a single note vibrato paying attention to it but to say when he was having problems focusing on it. The student started, and 10 seconds into it said ‘time’. Suzuki congratulated him, pointed out that it would be rare for a single note in music to be held vibrato that long. He asked the student to practice the vibrato on one note for 5 seconds with a timer a few times a day, but really focus on it. The next week they met again, and did the same exercise – and the student could fo it for 45 seconds.
Attention/focus is gained by doing it within your current abilities well instead of doing it poorly outside your abilities.