It’s quite common honors/accelerated students to take high school level classes in middle school, particularly among applicants to a selective college like Stanford. Different high schools have different policies about how they handle such classes. Some list them on the high school transcript and/or count them towards HS GPA/rank, while others do not. To have a more standardized way of comparing grades at different high schools, Stanford recalculates GPA using only core subject classes taken in 10th-12th grade, and has a history of making admissions decisions consistent with such a recalculation. I haven’t seen an explicit statement from Stanford saying how much weight they give to classes taken in middle school if they do appear on a submitted transcript, but their other comments imply it is not much.
For example, at http://admission.stanford.edu/application/freshman/transcripts.html , Stanford’s website states,
They make it clear they want a transcript for grades 9-12 and will focus on grades 10-12 . They make similar comments about grades 9-12 foreign language elsewhere, such as the page at http://admission.stanford.edu/basics/selection/evaluate.html using wording like foreign language in “grades 9-12” or “3 years foreign language through 11th grade” (grades 9-11). They use this type of grade level specific wording to make it clear they are talking about foreign language taken in grades 9+.
Speaking form personal experience, I also attended a school system that started foreign language in middle school. My school started in 6th grade. Most kids finished Spanish 1 by 8th grade, and accelerated kids finished both Spanish 1 and 2 by grade 8. I mostly received B’s in foreign language, so I was not in the accelerated group. The accelerated group completed Spanish 6 by senior year of HS, and I stopped the sequence after Spanish 3 (maybe Spanish 4, don’t recall) and instead took a Latin class that I enjoyed more. So I had less advanced foreign language classes than most at my HS and didn’t receive quality grades in many of the classes I did take. Nevertheless I was admitted to Stanford based on far more impressive accomplishments in the fields that I was more passionate about and planned to pursue in college. In short a single B in 8th grade is not going to have much influence on your chance of admissions, even if it is in a high school level class.