Regarding schoolwork, on one hand I think that an instructor should be responsible for his students, so if they were failing to do their schoolwork to attend training, I think he should intervene. However, what if you're just not good at learning and training is the only thing you do well?
My past teacher required that kids bring in thier report cards. But he wasn't so much concerned with grades as thier conduct. He realized that some kids are just not academic gifted, and was more concerned with conduct (that which they could control).
In the school I just joined, I think it is much the same. I know they list that the goal of all students should be a B average. But I think it is just that. A goal.
I do know that the kids do have a form they take to thier school teachers to fill out concerning thier behavior before they are allowed to test for the next rank.
They can still attend class, but can not test until thier behavior improves.
Disipline in class:
As a Youth coach. Your safest beat is to be very clear to the parents of your kids, what you expect from thier kids while in your care. You don't need a long list of items like "for offense x the punichment is Y"
Just let them know that punishments for disrupting class, can include exercise, sitting out, not testing until behavior improves, ect.
While you hate to lose a kid (usually because of parent). You have to weigh keeping one kid. With disrupting the learning of all the others present.