When my DS "pencil whips" an assignment, I simply make him do it again. He's learned over time tha it's much quicker to do it right the first time. Ask the teacher for duplicates for his worksheets. If he doesn't do them right, hand him a new one and make him start over. Don't let him correct the original, but rather start back at the beginning. If he refuses to do the new one, let him know that there will be no reading, video, tv etc until it's done. Don't argue, just give him the rule, the new page and walk away.
It took a little time, but it worked for us.
We do the same. I don't push for correct answers as much as I push for neat work and good behavior in class (another issue for us when DS6 is bored). When he takes his time, he writes better than I do! And my DS is obviously young, but he gets frustrated when I can't read something he wrote, so that helps keep him a bit neater... of course, he still tries cramming words into the margins and/or on the last (non) line of a page.
