Download High School Reading List Books in the Public Domain as PDF
Download Middle School Reading List Books in the Public Domain as PDF
I cross-checked High School and Middle School Reading Lists with books available in the public domain and came up with these lists, using my handy-dandy Aunt Lee's Library Search Engine.
These are all available for free; one or two are only available as PDF's, not e-books. Feel free to re-distribute these to teachers and students -- there's a checkbox next to each book if you'd like to personalize selections. Encourage students to use the Aunt Lee's Library search engine, which only goes to good, free, e-book sites, many of which also have descriptions and reviews of the books. It's also a good idea to go to http://www.goodreads.com/ and search reviews there.
Kids nowadays are accustomed to downloading pirated e-books from illegal torrent sites; you may want to remind them that this is illegal, that it's far better to save money by reading books truly in the public domain and then spending their money on books still in copyright.