I've posted on teaching and testing writing before. You can read those posts here and here.
Here is a column from Inside Higher Ed about two colleges' approaches to improving their students writing skills.
I will be honest, I never got anything worthwhile from writing tests. When I was at Harvard, we had to take a writing test our first year, before classes started. If you passed, you got to go into the regular freshman expository writing course (small class size, four papers, consultations with the instructor, etc.). If you didn't pass, you had to take a different writing course, then you got to take the regular one. At my next school, they wouldn't let me out of English I and II despite the fact that I had a bachelor's degree from Harvard. I didn't get much out of this class because my peers were all 18-year-olds fresh out of high schools in a major urban center. The instructor was fighting a losing battle. Some time after you'd completed English II, you took a writing test where you picked up a packet of articles, read them, and then wrote essays about them; I don't remember if there were specific questions or if you could write whatever you wanted. After that, you had to take a writing-intensive class in your major. For me, that meant TWO writing intensive classes, because I was a biology and chemistry double major.
The chemistry writing class wasn't so useful; they just took our physical chemistry lab and made us write extra-long lab reports. Not much emphasis on writing quality except if the instructor saw really stupid sentences, he'd read them to everyone (without telling who had written the sentences; luckily, none of mine ever got that sort of attention). I actually thought the biology writing class was useful; we had an exacting Englishwoman as an instructor in a very small class, and I learned a lot there.
Where I'm in grad school now, there's not a real English major, and there's not a real English department. Students gripe about 13-page reading assignments (no kidding, I overheard a long discussion of this on a campus bus yesterday) and writing papers that are shorter than 5 pages in length. I frequently want to smack them and tell them to shut the hell up. And when instructors do assign term papers, the writing quality is just abysmal, as I've had the opportunity to see some of these as a TA (thankfully, I did not have to assign grades to them). It's kind of a joke about the particular school I'm at, that the graduates can't write. They may be darned smart, and good engineers or whatever, but their command of the written English language is subpar. I would like to stick them in a government class at Harvard with a 20 page term paper and a couple of hundred pages of reading a week and watch them flail.
I've gone off on a rant here and have forgotten my original point. I think what I wanted to say was that writing instruction needs to be integrated throughout the curriculum. Just because you're in a class called "biophysical chemistry" doesn't mean you don't need to think past problem sets. One course I was a TA for required students to choose a paper from the scientific literature, summarize it, and provide commentary. This called for reading AND writing comprehension, and is a good way to incorporate language arts into a science curriculum. It shouldn't be "one class and you're done." Every class should require writing in some form, after a couple of introductory courses devoted to technique.