Instead of lowering your standards for students with learning difficulties, give them longer to achieve writing competence.
You may also need to break work into smaller segments to allow struggling writers to complete it.
Even the best teaching practices for writing often must be supplemented by instruction in time management, self-assessment techniques, and positive self-talk. Struggling writers almost always need help in one or more of these self-monitoring tasks.
Ordinary students can usually benefit from beefing up their skills in these areas, too.