P.S. I meant to point out the irony that English classes contributed comparatively little. Actually writing about something and putting in the effort to string together something coherent is what really exercises and builds writing abilities. It's nice to have some of the groundwork laid, but practice is by far the most important component.
In 6th grade and beyond, the school went to a more traditional math curriculum, I suspect due to milestones such as getting through algebra in 8th grade, etc. For the kids to do more writing in math class requires a choice of what they will spend less time on. Do they learn fewer math topics, get fewer assignments in Social Studies, or get less sleep?
Maybe I'm a freak, but as a student I greatly enjoyed the "pure" nature of math. In other subjects, I found that I could get good grades by basically filling pages with drivel. On the other hand I didn't just write down solutions. I was expected to show a derivation, and in many cases, a proof. I have a copy of my high school pre-calc textbook, and a large fraction of the chapter problems are proofs, which I loved. That's what motivated me to become a math major in college. Today, the proofs have disappeared.