Squibler is not available for free past a 14 day trial mode, but it does have some free features that you and your students can use without an account.
The Plot Generator helps you start a story with an idea. Students could copy the idea into their own document and work on their story from there. The site also offers The Most Dangerous Writing App with plot generator. You can generate a prompt and start writing on the site. However, if you stop writing, the editing session will end. The site does give you a chance to download what you have written, unlike another writing app I’ve posted about.