What worked well for us was to have a structure and routine to the day and to have school included as a specific but small part of it.
On average both DS6 and DS8 spend a little more than 3 hours/day on school. That doesn't include independent reading and outside classes (piano, swimming, piano practice, etc). We have a specific schedule for each day and try to fit most of our school work between 9 and 12.
It has been working really well for us, but this is already my 3rd year of hs. It takes time to find the right rhythm for everybody.
