That's why we have to get out of they're way. Get out of their classroom. They already have all the skills necessary to learn how to write well. All they need is an environment whereby they can practice. And that means conversation.
Conversation develops fluency.
They already speak fluently. So what we need to do is immerse them in written conversation. Not written conversation with the teacher, but with people they feel they can persuade of something. Where does such an environment exist. On the Internet.