Leah,
Julie gave you great advice! I agree that the writing skills within Rod and Staff will be so important, along with the Medieval Writing Lessons.

I would likely lean toward doing Rod and Staff more slowly if needed in order not to get too heavy in the writing area, rather than skipping the writing lessons within Rod and Staff. Perhaps, you could do Rod and Staff English 6 three days a week and then throughout high school do a 3 times a week grammar plan instead of a two times a week plan? While we encourage families to strive to complete Rod and Staff 6 by the end of grade 8, it is only a hopeful guideline to shoot for and may not happen for everyone. I would always lean toward balance first, making sure not to overload a child in the language arts area, while still getting a well-rounded language arts education.

Secondly, I would strive toward doing the lessons in order, rather than skipping, as there is much meat in Rod and Staff and much thought in its step-by-step design.
If, for example, your child does not complete English 6 by the end of grade 8, I would just continue on in high school, thinking of it as a continuum. In that mind-set, however far you get into Rod and Staff English 7 and 8 is a wonderful benefit most high school students are not currently getting!

Even if your student didn't make it all the way through English 8, he/she would still be getting an excellent English foundation through Rod and Staff.
Blessings,
Carrie