Teachers and students will love their ability to comment on their work and the work of their classmates. When you have "edit" or "comment" access to a document, you can easily add a comment (like a post-it note) right on the document. All you have to do is select some text and click the comment button at the top of the page.
You will be able to type a note to the owner. Then your comment will show up and an email will be sent to the owner of the document.
The owner will then be able to go into the document and resolve the suggestion. Also, take notice that you can +emailaddress another person to add them to the conversation. When you + someone, they will get an email, and you will be prompted to alter the share settings for that person. You could add them as an editor, commenter, or just allow them to reply to the comment. Interesting possibilities there.
Comments will be an excellent way for teachers to help students edit their work, without having to edit the work for the student. Make suggestions, provide hints, or even create a class code for editing tips. Give comments a try.