The teacher posts an assignment, perhaps including special instructions or even a sample paper.
Students write their papers, using any program.
They can read and comment on other student's papers, and may copy and paste from a student's paper in their comment, thus being quite specific. The teacher can mark text and type comments which are hidden annotations in the student's text that a student sees by double clicking on a small icon. Or the teacher can easily link an icon to an applicable section of the on-line handbook. I am still learning just how powerful this tool is. Students seem to take their writing much more seriously when they have to present their ideas to students as well as the teacher, making their ideas stand up to questions and different reader perspectives.