i wouldn't say they don't matter until your in high school.
If you perform bad your 8th grade year you are going to end up being recommended for low level classes in 9th grade, and by then its usually too late and your entire core curriculum is already chosen (math, sciences, history english).
When I say perform bad I mean borderline fail, not getting all C's.
Your grades all of high school matter, take it from me I slacked all my freshmen and sophomore year not realizing the effect it had on my gpa until my junior year when I started to look for a college. It was never expressed to me how important all of it was until it was too late, now im stuck in a community college next year until I can transfer.