According to recent research, explicit grammar instruction doesn’t improve writing skill, but more time spent writing does.
To teach kids to write well, you need to ask them to write a lot. You’re not going to become a great basketball player unless you play a lot of basketball. The evidence is strong that this is true for writing too. Five studies of exceptional literacy teachers found that great teachers ask their students to write frequently. In nine separate experiments with students, 15 additional minutes of writing time a day in grades two through eight produced better writing. Seventy-eight percent of studies testing the impact of extra writing found that student’s writing quality improved.
Several studies found unexpected bonuses from extra writing time. Not only did writing quality improve, so did reading comprehension. Another cluster of studies proved that writing improves a students’ mastery of the subject; the act of writing helps you learn. (Another reason for teachers to refrain from spoon-feeding printed notes to students.)