People learn the skill of writing through repeated practice in writing. Learners may be helped by reading about writing or through isolated practice of some tricky writing technique, but people learn to write only by repeatedly going through the entire process of writing.
The best way to provide practice is to have students respond to expository nonfiction writing prompts (called essay questions when I was in high school) across the curriculum on a regular basis.
To guarantee enough practice, students from middle school through high school should write an essay at least every other week. The younger students can write single-paragraph or three-paragraph essays; those in tenth grade and older should write five-paragraph essays (which need not have just five paragraphs) requiring at least two good reasons for believing their thesis is true. Under that kind of regular practice regimen, students will get so they can produce a competent piece of writing with only modest effort.
Teachers should allow students to decide which essays are important enough to warrant more than a modest effort. (Be honest. How many days a year do you feel you are doing something on the job that’s worth your all-out effort?) If teachers do their jobs well, each student should find at least one essay a year that she or he feels is worth putting real effort into doing well.
If students are really writing across the curriculum, teachers in one specific subject area shouldn’t be stuck giving all the feedback and doing all the formal assessments.