I've not yet read or corrected their work. But I pretty much know what I'm going to get, because I've had my students for six months already. Many of them are fairly gifted writers who enjoy the process. Many others hate to write and do it very poorly. Becoming a better writer takes time.
Progress is incremental.
Repetition is essential. Epiphanies are few. All writers, especially struggling writers, need full and thorough feedback. As a newspaper editor, I would often sit next to a reporter for two or three hours combing over a story, discussing it, polishing it. As a teacher, though, I bring home these 100 paragraphs, full of both flaws and potential, and spend two or three hours reviewing them, marking them up, grading them.