Generally, a higher ranked school indicates a stronger cohort. However, I think the most important thing is a strong group of students at the top of the cohort. The closer to rank 1 you are within your own cohort, the better it is, irrespective of how well the cohort as a whole performs.
You can go to a extremely low ranked school and yet if you are rank 1, it doesn't matter. You still have the same chance of obtaining a 50 as another student who attends a top tier school. The strength of a cohort largely comes into play the further away you are from rank 1, as this means your scaled SAC score can be quite unpredictable despite how well you do in the exam. This is especially true with the Englishes where your (scaled) SAC scores comprise 50% of your study score.
This is just my superficial understanding of rankings/moderation and I'm sure there are better, more detailed explanations in this forum (I know for a fact :p).