I just skimmed your letter and I bet the school did too. I'm doing advocacy now for my 2e preschooler and everything I've read advises keeping letters under a page if at all possible. Long letters have a tendency to be perceived as being whiny regardless of how well written.
Lots of kids take 3-5 AP classes a year in HS so I think your basic argument is weak. Schools view balancing extracurriculars as your problem.
Advocacy wise, I would try to find an online class or independent study that the school would accept in lieu of the course. This allows the school a graceful out and addresses any curricular concerns they may have.
Good luck,
-chris
