I disagree with the claim that Basis weed out weaker students purposely. Obviously if your kid is under performing at the school, as a parent, you should look at a different school that better meets your child’s academic abilities. Just as I had to do when the public school system wasn’t meeting my children’s academic abilities.
I did look at the recent USNWR score and thought it was a little suspect, but Basis live up to my expectations of delivering the kind of education I want for my children, so that is as far as my concern went.
As far as the drop in students from lower grades to high school. A lot of students decide to go back to public school during high school, because they want “the high school experience,” whatever that means and that could account for the drop in students that you see at the high school level. I don’t think that shows anything nefarious on Basis part, the can’t force students to stay.