I don't start watching movies because I want to write a nasty review: I watch them because I hope they'll transport me to another world, provide a sense of wonder. I'm not a paid reviewer, so I only watch movies I want to watch. And this isn't a bad movie, in fact it's reasonably good ... but I think it's fair to call it derivative.
Imagine Kiki (of Hayao Miyazaki's "Kiki's Delivery Service") goes to evil Hogwarts, with a big serving of every other Miyazaki film on the side for visual flair. This is a very visually attractive film aimed at children, and the children may not notice how derivative (or how earnest) it is, but adults familiar with Studio Ghibli's output and the Harry Potter films are likely to find this awfully familiar. It also suffers in comparison with "Your Name" by Makoto Shinkai, another Japanese Anime movie I saw earlier in the day - "Your Name" was even more impressive to look at, and distinctly more original in its ideas.
A short plot summary: Mary Smith goes to the countryside to stay with her great aunt. A cat leads her into the woods to an unusual flower, which grants her (although she doesn't realize it immediately) temporary witch powers. A broomstick flies her to a school in the sky, where she's assumed to be a new student. But not all is as it appears (and I've already given some of that away by saying "evil Hogwarts"). I imagine that this would work very well for kids, but fans of Miyazaki shouldn't rush to see it.