I don't know how that test compares to grade level. It seems like a test to determine one's CEFR level, 20 correct answers out 20 gave me C1. It doesn't really say what it's based on, but it does encourage you to buy their product.

Ok actually, this seems to suggest B2/C1 is about 6-8th grade level and C1 is 9-12: https://wida.wisc.edu/news/wida-model-online-scale-scores-linked-common-european-framework-reference-cefr