Showbie Limitations with Editing Google Docs.

I really like using Sowbie for students submitting work.

Problem: Although I wanted to use if for the purpose of drafting into a closed task, and it isn't possible for students to edit pdf or word docs. 

Solution:  If the Google doc is submitted by teacher into Showbie as word doc. and opened by students in Google Drive, then the students can edit and resubmit the final draft from Google Doc. to Showbie. This way my students don't need to be a signed up for Google classroom.