Depending on how big you might want to get, there are a couple of options, one being the Journal plugin. You can add comments and students can respond, or a student can write something and you can respond. This is an online activity only, has only one page, is accessible only by one student, is updatable, so may be of use. If it gets too big though, it can be a pain as you are constantly scrolling to find previous comments.
You may also choose to use a forum that has only one subscriber, the student, but you will need one per student, lot of work. There is also the option of just using an online Assignment. They edit the assignment an unlimited number of times, to which you respond with the comments you need to make, which leaves it open until you close it, after completion of the course.
There is likely to be another plugin I don't know about that might help here, so you might want to look in Downloads > Plugins.