"You know how you spent $2000 on that trombone for Little Johnny? Well, the AP has decided that they don't get to play it because they missed an assignment in Science class. The AP is pulling them out of band class to complete missing work."
The problem will solve itself in about a week.
Seriously: pullouts may not take place without the express written consent of the parents. Period.
Also: brush up your resume. You're going to get non-renewed for "not being a team player" because of this.