Story Wars is a collaborative online workspace where stories are written, one chapter at a time, by different authors.
This website is great for two reasons: 1. Loads of books, and 2. The ability to create your own. Unfortunately, the quality of the books vary, so you’ll want to vet any stories before recommending to your students. There is a paid tiered that gives the teacher more control and the ability to curate stories for your students and moderate stories. Also, the paid tiered will keep your student’s stories private, which may be a requirement depending on your local standards.