I talk regularly about how I think that kids must, and I mean MUST, write in the math class. It's a lot of work for you as a teacher because you need to give time to do something lengthy, and then give some time for them to start the write-ups during class, then you need to give them copious written feedback on the first draft, and then maybe have a discussion with the whole class about how to organize their ideas logically, and then you need to still read every kid's final draft and to assign them grades, and then to post a task-specific rubric. In the end, this process is so SO worth it, even if some of the kids' products are not stellar. The important thing is that when they write, they are really learning about what it takes to put their mathematical thoughts down in words in a way that makes sense to another person outside of our class, and they're forced to re-evaluate how well they themselves understand each step. Moreover, if they are asked to make a draft and then a heavily revised draft, they begin to invest in the quality of their work and to learn to take pride in their work -- no matter how much help they might have needed along the way.
Here and here are two lab write-ups from earlier this semester (maybe November), when the kids wrote about the process of stacking cups to match a classmate's height. The two kids I posted are just two random kid athletes who happened to send me their files before they went away on a sports trip, so they definitely weren't the most polished pieces from the class, but they ARE a realistic look at what "regular" 8th-graders can do with a bit of support. (These have some errors in them, of course, but I decided to just post them as is. The kids who had really top-notch, exemplary projects didn't submit digitally, and I haven't bothered to scan them.)
I also have some interesting samples of the percent/shopping projects and the three-variable projects that my 7th- and 8th-graders worked on at the end of Semester 1, leading into Semester 2. I'll post those in a few days if you check back then. They've turned in their "first drafts", which they think are polished (last graded assignment of Semester 1), but look fairly hairy to me still, and now they'll get a chance to polish those drafts based on my written feedback, for the next round of submissions (counted as part of Semester 2).
Hope this makes you excited about writing in the math class! I love projects and I love making kids write about math. For me, I think all worthwhile math projects are inherently interdisciplinary, because even if you're teaching strictly math, as soon as the kids are asked to communicate, reflect, present, create... then you're already bringing in essential skills that they develop from other classes and helping them see that other subject skills do not stop at the classroom door. After all, we are full-time educators as much as we are math teachers. :)
PS. I have to admit that sometimes, grading the first math writing assignments of the year makes me very aggravated, because most of the kids have never done such math tasks before my class. They don't know how or when to use diagrams to visualize; they jump straight into math discussions without any introduction; they either write a lot of symbols without attaching any description OR they just give numerical results without showing the work on how they got those values; they keep repeating themselves because they cannot organize their thoughts in an efficient manner; and they seem to have developed some secret language that doesn't make sense to anyone outside of the class but me, in describing their mathematical process. But, THAT IS EVEN MORE REASON for them to write!!! By the time they leave my class, I would say that the majority of students can piece together a fairly coherent discussion of a relatively straight-forward math topic, without too much help on my part.
I just wanted to put this out there in case you are wondering, "Is it just my students who are so bad with mathematical writing?!" The answer is No. Usually I have to eat a lot of chocolate and to take lots of rest breaks when I grade the first writing assignment "rough drafts" of the year.