CTY offers Crafting the Essay
http://cty.jhu.edu/ctyonline/writingcourses/cdw3andema3.html which is excellent! The instructor's feedback was almost as long as my son's essay each week. They were color coded for needs improvement, positive feedback and neutral comments and were usually pretty balanced between the three colors which encouraged improvement while building confidence.
The Web-based version gave a real classroom experience by requiring the kids to participate in on-line discussions and provide useful criticism on their classmates writing. They also have a social area called Pizza Party where the kids can have non-course related discussions. My son took this class 4 years ago and still maintains contact with several of his classmates.
