Convenient access is a huge deal. Students are at school every day whereas with public libraries they are limited to their parents' schedule. I know I was able to read nearly twice as much fiction as a teen because I could checkout a new book the same day I finished the previous one rather than waiting a week or so until we could go to the public library. Having the library is also useful for integrating into classes, for example my English classes would alternate between assigned books and reading a book of your choice, and we would make trips to the library during class time to be taught how to use libraries for research. Lastly, in rural areas the school libraries are often the only library.