Collection development is essential to attracting (and keeping) readers. Your collection must meet the needs of your patrons... in our case: middle and high school students. Keeping up with new trends and new releases requires more than just reading book reviews in professional journals. A great librarian will get to know her readers and what they like.
Collection development is also more than just buying great books that circulate frequently. It also means knowing when to dispose of, or "weed," the old stuff as well. It also means knowing what your readers need in to electronic formats and what you need to keep in print.