Storium is a free collaborative storytelling platform that has just opened itself up to the general public, and which I learned about from my colleague AJ Sisneros.
Unlike most other tools on The Best Sites For Collaborative Storytelling list, Storium lets you choose who you want to write with, which obviously works best for teachers and students. However, it seems that many of the free “worlds” you can use in the stories can only have a limited number of writers (you have to pay to increase the numbers).
The video below explains it more in detail. It’s probably too complicated for English Language Learners, but I think it could have potential for other classes.