School libraries are structured to support the curriculum, whereas public libraries contain a more general collection that meets the needs of a larger community.

For example, a public library might carry a few books on say, literary criticism, but they might have one for every major area: American, British, Modernism, non-European, etc. A high that has English classes concentrating on British and American literature (mine did at any rate - Am. Lit my Junior year and British my senior year) will concentrate on literary criticism in those two areas - because that is what the students will be studying.

It's not only for pleasure reading, but for research. Most high school (and college and university for that matter) libraries are going to focus more on the non-fiction research collection than the fiction collection, except where that supports the curriculum, like Accelerated Reader books.