1. Admit almost anyone to the sixth form (money follows the pupil and you need all you can get).
2. Set tough exams at the end of lower sixth, to weed out the weakest lot - if they don't pass they can't continue (but you've got the money for a two-year course, so that's OK).
3. After mocks, don't enter anyone who's "only" going to get grade C or less (you don't want to bring down your league table position, do you?). Of course he or she can always enter privately, but private entries don't appear on the school's results.
As a head of sixth form from 1989-94, in a school which does not operate like this, I observed this system developing and saw students' careers being permanently damaged. It is the inevitable result if you put together: a) money follows the pupil; and b) judgement by league tables.