Peg Tyre considers how learning to write has changed:
Fifty years ago, elementary-school teachers taught the general rules of spelling and the structure of sentences. Later instruction focused on building solid paragraphs into full-blown essays. Some kids mastered it, but many did not. About 25 years ago, in an effort to enliven instruction and get more kids writing, schools of education began promoting a different approach. The popular thinking was that writing should be “caught, not taught,” explains Steven Graham, a professor of education instruction at Arizona State University. Roughly, it was supposed to work like this: Give students interesting creative-writing assignments; put that writing in a fun, social context in which kids share their work. Kids, the theory goes, will “catch” what they need in order to be successful writers. Formal lessons in grammar, sentence structure, and essay-writing took a back seat to creative expression.
The catch method works for some kids, to a point. “Research tells us some students catch quite a bit, but not everything,” Graham says. And some kids don’t catch much at all. Kids who come from poverty, who had weak early instruction, or who have learning difficulties, he explains, “can’t catch anywhere near what they need” to write an essay.
Former 5th grade teacher Robert Pondiscio claims it is a mistake for grade school writing curriculums to value ideas and personal expression over grammar and mechanics:
Far from imposing a cultural norm or orthodoxy–silencing their stories and compromising their authentic voice–teaching disadvantaged children the mechanics of writing, and emphasizing evidence over anecdote, is liberating not constraining. Teaching grammar, vocabulary. and mechanics to low-income black and Hispanic students is giving them access to what Lisa Delpit, an African-American educator and a critic of progressive education methods, famously called the "culture of power."
Let me hasten to add that there should be no war between expressive writing and explicit teaching of grammar and mechanics. It's not an either/or proposition.
Further discussion on the topic here.