When students turn in work to Google Classroom the ownership of the document is switched from the student to the teacher. This means the student can no longer edit the document. Google Classroom allows the teacher to return the work to the students.
After turning in work, the student has the option to "UNSUBMIT."
When a student unsubmits, the ownership switches back to the student and the student can edit the document. A record of students submitting and unsubmitting is located in the submission history.
Comments in a Google Doc are only available to editors of the document. When a document is view only, the viewer can not see the comments. Any comments made by the teacher are not visible by the student. Since the student is a viewer and not a collaborator they do not receive email notification of comments left by the teacher.
Returning Student Work
In the Stream click on the assignment title to view the assignment assessment screen. There the teacher can view student work and provide private comments on student work. Leaving private comments in Google Classroom will send an email notification to students.
The assignment assessment screen also allows the teacher to "RETURN" work to the students. Check the box next to individual students or click on the "Done" checkbox to select all the students who are "Done." Click on the "RETURN" button to return work to students.