This is perfect. I beg you not to change it. I like the other covers I've see for movie posters, webpages, and the like, but this one I think perfectly summarizes most of what is important. The Vitruvian Patronus is BLOODY BRILLIANT!
Your instructor is mistaken about it being too busy- look at other Harry Potter covers and you'll see there's actually a lot going on in 1, 2, 3, 4, and 7.
I happened to like 5 the best, but that was only after reading it, and in my opinion 6 wasn't nearly a busy enough cover.
You are now my #2 fav. DeviantArt Methods artist, after Dinosaurusgede, and that's only cuz he's done so much. The covers have always been my favorite fan arts, and each new one is better than the last!
Please do a back cover, or see if you can collaborate with one of the other cover artists to adapt the two covers together. I think the Crystal Ball and Snapping Fingers would make a good back cover to this- anyone else agree?