Flip answer: the bucket width should be 2.5 times the score improved of a prep course.
Obviously, if a school has a cutoff score bucketing is easy, but with excess applicants ordering becomes necessary. I guess this sort of probabilistic score would induce an order for any given student relative to sufficiently superior or inferior applicants.... I'm now kinda curious to figure this problem out. Did not expect an algorithms problem to arise in this thread lol