I get asked this question about twice each week, I really do, and my answer always sounds inadequate because the homeschooling curriculum world boasts dozens of writing programs, most of which are probably quite good and quite similar. It's hard to say that one is better than another because format preference largely depends on the student's own learning style.
The goal of any writing program should be that the student can organize her thoughts efficiently and translate thoughts to sentences. In order to do this, the student needs a structure on which to attach these sentences, and the most common structure is the 5-paragraph essay.
By the time a student finishes high school, he should (if I were a “must” kind of person, I would say “must” here) be able to write a fluent 5-paragraph essay. I am not, by any means, saying that this is all he should be able to write, but…the 5-paragraph essay is the foundation of all composition. If she can master this type of essay, she can master a research paper and ultimately her dissertation.
The five paragraph essay follows a specific format. The introduction (first paragraph) introduces the thesis (topic sentence) of the essay and its three main supporting subtopics. The body (second through fourth) paragraphs individually restate the subtopics, one in each paragraph, and provide supporting details. The concluding paragraph restates the thesis and reminds the reader of the three main supporting ideas that were developed.
You don’t have to buy a book to teach the 5-paragraph essay. The internet has abundant resources. You can take any one of these websites and take a semester to teach focus on writing good, solid essays of various types: expository, narrative, persuasive (teens love this kind especially). Start with writing the introduction and work on that introduction until your student can quickly churn out the three subtopics. Then begin working on the body paragraphs, making sure that they see the clear connection between the three subtopics listed in the introduction and the three body paragraphs. Following are some excellent resources that teach the whole process (or wait for my e-book, which is one of my Projects):
The Five Paragraph Essay
Five Paragraph Essay
Essay Writing Center (be sure to look on the sidebar for links to different types of essays)
Guide to Writing a Basic Essay
Education World (has SAT specific information)
Brain Pop (goofy but contains a cartoon that kids might get a kick out of)
English Comp (gives you an idea of just how much focus is given to basic essay writing at the freshman college level)
If you spend a whole year perfecting the 5-paragraph essay and its various types (descriptive, narrative, expository, persuasive, etc.), you'll have accomplished much of what is covered in a basic freshman composition class. Imagine how far ahead your student will be if he is familiar with this format in middle school and fluent by high school!