I know that your post is incredibly old in a thread that's been around for a while, but I absoutley had to quote you... because SCHOOL DAYS IS A GREAT ANIME!!!
I will never be able to understand why people don't like it. It's a geniusly told story about the loss of innocence, yeah sure it's really dark and a little over exagerrated, but it has that incredibly deep impact that explains the darker side of love, lust, and relationships.
So good job TyrantMigs, you have great taste!
As for my personal favorite anime with 13 or less episodes, my number one favorite would have to be FLCL.
I have a few others as well but can't think of all of them at the moment.