Well, not all countries use percentiles, either. Finnish college grades are a number on a scale from 1 to 5 (and high school grades are a scale from 4 to 10), and it depends somewhat on the course and subject how those grades are produced. Some courses, for instance, will pass you if you get 33% of the exam right, others require 50%. In either case, only the grade is recorded, not the percentile.

Rural vs. urban is probably a pretty good control for the license question.

No ideas that I could think from the top of my head, but I'll comment if anything occurs to me.