You can play both but can not miss a practice or game per miaa.
That should be put at the top of the "Hockey Parent's Gospel" or things not to do list.
A prior poster mentions "making varsity." For a U18 player, that likely isn't an accomplishment. I have to think if a kid is thinking about playing for a full season U18 or Junior team, their HS team is not strong. We aren't talking U14 here.
If a parent wants to narc on a kid that is trying to help them to be better, it only hurts the team. Sitting out HS games and practices is putting the kid back in the same position they would be if they hadn't played - playing solely for their likely better U18/Junior full season team.