Project Gutenberg has a collection of more than 68,000 free downloadable ebooks, in their online catalog. You can search the catalog by author or title. You can read the ebooks in many formats including online, ePub, Kindle, and plain text.
Some titles include collections of versions for younger children as well as older students. Aesop’s Fables is an example.
Please note that many titles have copyright restrictions in how they can be used beyond personal reading, such as for display or copying and distribution.