Aside from quantization errors for high school classes with fewer than 16 students, why would it be easier to get the top 6% in a rural school?
In an urban area, residential areas are typically segregated by wealth, and wealthier parents want their kids to go to good schools. Further, they are capable of moving within the area so that their children go to good schools. As a result, the competition among students for the top N% more fierce due to the pre-selection of students.
I think making the system more equitable by supporting kids from all backgrounds is socially beneficial, even if it sometimes makes suburbanites complain.