Storillo is a writing tool for group work. Think of it as Google Docs version history on steroids. You can track each student’s contribution to the document in an easy to use format.
The teacher can look at the finished document and see exactly which student did what:
The site is free for 3 classes/year and 2 projects/class. Pricing starts at $60/year/teacher for unlimited classes and projects. They do offer school pricing, but you have to contact them for a quote.