I like using both high-tech and low-tech tools to help students create collaborative stories (see The Best Sites For Collaborative Storytelling).
There’s a new addition to that list now that Story Wars has created an option for people to create private stories where only invitees are allowed to contribute.
The private option is really the only viable one for a classroom; when it’s a public version, anybody is allowed to contribute.
Assuming the site makes it past school Web content filters, it’s worth exploring….
Addendum: They’ve also created a Classroom feature that is unfortunately pretty pricey – over $25 per month.