What consequence is there for missing sectionals? A dropped grade, perhaps?

What consequence is there for missing practice? They run laps (to make up for "missed conditioning" which we all know is bullsh) and could be kept out of the game for an extended period.

You need to speak with your athletic director and head coaches and come up with a policy that makes it activity-neutral for a kid to miss practice for one event over the other - and yes, that means you may have to change your policy too. Then hold those coaches accountable to following the policy.

Band is going to take a second chair to sports in 99 of 100 towns. You could also consider moving sectional times to better accommodate the athletics schedule. If they have practice after school, have sectionals before school.