You really can’t include it if you are using the Common App. When you list an honor/award, you are forced to choose the grade level in which you achieved it. The lowest grade than can be chosen is 9th. This is true for your activities list, too.
The other reason you shouldn’t list it even if you could is that colleges want to know what you are doing more recently. If you can score 16/10,000 in middle school, how have you built on that math aptitude? Are you still ahead of your peers, or did you peak in 8th grade? Are you continuing to pursue mathematics and shine in that area, or was this a testing fluke?
Middle school matters, but in the way they it prepared you for high school, not in what you pursued or accomplished at that time.