As an administrator, I cannot be the content expert on EVERY single course my school teaches. I don't know AP Biology. I don't know the ins and outs of the updated Precalculus TEKS, even though I took that class in the long-long-ago. I couldn't tell you a single thing that goes on in an Engineering class - but I could be responsible for any or all of those teachers.
TEACH your principal what beginning band looks like. Teach them the process. Invite them to sit in with the class. EDUCATE. That way, next year, when they ask "Why can't we just make 7th graders meet during 6th grade periods?" or "We can double block your beginning percussion in with your advanced band, right? You have two teachers!"
or some other equally silly idea, you can refer BACK to the work you're doing NOW and explain how the curriculum and scope of the course is different.