In 1963, a high school student wrote to famous authors, including Ray Bradbury, Jack Kerouac, Ralph Ellison, and Ayn Rand, asking their opinion on symbolism in their work. This site has scans of their answers. It's really interesting if you've ever wondered, "Did he REALLY mean for this to represent THAT???"
To use the Hemingway App, simply paste your text over the sample text featured on the page. The application will highlight what it identifies as "hard to read" and "very hard to read" sentences, as well as occasions of passive voice. While papers for school should not necessarily be written below the tenth grade reading level, the application might be a good way to check papers for passive voice, wordiness, and syntactical fluency before submitting them.
Either way, it's fun to play around with. (These directions are supposedly a twelfth grade level.)