BoomWriter is a creative writing website. It works by teachers creating a story starter and choosing how many chapters the story will have. Then students continue the story starter and write a chapter of the book. The chapters written by the students are then submitted anonymously and are then voted on. The winning chapter is then added to the story and the process continues until the story is complete. Once the story is complete it can then be published.
You can sign up your school and then encourage students to sign up. It would be nice if they had included a better way to sign up your class from a class list rather than waiting for students to sign up individually. Below is a short YouTube clip explaining a bit more about BoomWriter.