For this functionality, your teachers will want to use the Advanced uploading of files assignment type. The upload a single file assignment does not offer a simple solution for returning a file to the student. For more information on the Advanced Upload type see http://docs.moodle.org/en/Advanced_uploading_of_files_assignment
If your teachers already have submissions to assess in the simple upload assignment type, then the best option might be to link to the graded essays through the HTML editor in the feedback area on each submission. If you need additional guidance on how to do this, let me know.
Then, make sure to change all future assignments to the Advanced type to make the grading process easier on upcoming assignments.