Forget the cliché.
Practice, even lots of practice, doesn’t make perfect.
The concert pianist practices even after performing for international audiences for decades: Practice hasn’t made her performances perfect.
Practicing won’t make your students perfect writers either, not even if they practice correctly very hard for a very long time.
But the goal of practice is not perfection, whatever the cliché says.
What does practice make?
Practice makes automatic.
Practice makes competent.
Practice makes efficient.
Practice makes permanent.
Practice makes proficient.
Practice makes plastic.
If you are teaching writing, those are the learning objectives you should expect to achieve by having students practice.