Here's a quick tip for organizing a classroom library--something that I just learned by trial and error.
One thing that is always a problem early in the school year is that fourth graders often pick out the second, third, or fourth book in a series, instead of the first.
In series for young readers, this isn't such a problem. I know this from vast experience of reading the Magic Treehouse books aloud to my youngest son! But books meant for older readers are often more complex, with each book building on the last. A reader who jumps into a series with the third book will get confused. (This isn't to say that it's impossible--I've done it, and I know lots of other readers who have. But it requires a different kind of reading, a knowledge of the genre and how series books work that's often beyond the scope of fourth grader.)
When students pick a book that is later in the series, I often have to tell them that this isn't such a great idea, and we go to find the first book. Invariably we find that the first book in the series is already in someone else's hands, which leads to disappointment. Take the Dragon Slippers series by Jessica Day George. The second book, Dragon Flight, looks beautiful and inviting. But it will be hard to understand without reading Dragon Slippers. And the kind of reader who doesn't know enough about series books to realize that it's best to start with the first is the kind of reader who just can't jump into the middle of a series.
I've solved this problem by pulling the later books of some more sophisticated series and keeping them on a special shelf behind my desk. Kids have to ask for these books. When I see that a student has been carrying around Dragon Slippers, for example, I know that she is going to be ready for Dragon Flight soon. My library shelves are less cluttered, and I don't have the problem of readers trying to read books that just aren't quite right for them.
In short: Put out the first books in the series, but keep the later ones out of general circulation. They'll be easy to find when you see that a reader is ready for them, and kids won't pick them up by accident.