I’m with everyone else I think the best thing an 8th grader can do is get in the habit of reading. It really doesn’t matter what you read - fantasy, sci-fi, mysteries are all fine - you want to become a fast and fluent reader. IMO the best way to learn grammar is to study a foreign language.
For what it’s worth I’ve heard good things about this CTY course - you have to be eligible to take it however - https://cty.jhu.edu/online/courses/arts_humanities/sample_essay_critique_writing.html I’ll say right now, I would never have forced my kid to do this. My older son chose to do CTY courses in mathy subjects because they were fun for him, my younger son went to a traditional sleep-away camp in 8th grade and music camp in 9th grade.