What do you mean by bad grammar?
Is it missed punctuation, capitalization (mechanics) or more of poorly written (thought out) sentences?
My oldest is in 5th grade and I have to say over the course of this year her mechanics have really come along as well as her sentence structure (thought out sentences). Of course room to improve, but I have seen a lot of improvement this year, so for her I think it was maturity and time for it to naturally develop.
We are using ULW and I really like the writing rubix that are included so that my DD has more direction as to what to focus on when she is writing. She can not focus on everything at once, but having those guidelines help her as she reads through what she has written.
As far as spelling, she is not a natural speller, so I do give a bit more “grace” for spelling and encourage her to ask for assistance. I am also not a natural speller so I can really relate to that struggle.