I disagree with Nancy's. Kolln conclusively argues that Braddock, et. I agree with Nancy that exercises and drills in formal grammar do not produce measurable improvement in student writing skill, so far as my classroom experience extends. But I agree with Martha Kolln that it remains to be proven that grammar should not be taught. I agree with Kolln that it is helpful to teach students enough grammar or language so that they and you with them can discuss what it is they have done when they have written well, or miswritten.
Language study is valuable in and of itself.
Properly taught, it can help students employ consciously their unconscious knowledge of language structure to their writing and thus make more effective use of that knowledge when they apply pen to page to improve sentence variety and style. I find that as students do frequent and extensive writing, they discover their own voice, and grammar and spelling problems tend to correct themselves, even when I draw little or no attention to them. Learning to get their point across effectively seems to take care of the rest. As to which grammar, if any, should be taught, I vote for structural grammar as explicated by James Sledd, Dr. Donald J. Lloyd, and others, as being the most student-friendly and applicable to the classroom. I introduce other grammars from time to time if the interest and need of my students so warrants.