Your definition of success is?
If a comprehensive raises a D-predicted pupil to a C (or better), it counts as a success for the school, and the pupil. If a B-prospect candidate gets a C or an A, then it's irrelevant either way as far as the school is concerned, because in the statistics it counts as part of the magic A*-C band. For the individual pupil, it could be catastrophic failure, depriving them of the chance for progress to A level study. Statistics are very blunt instruments, especially with grade banding.
Teachers concentrate understandably but disproportionately on the borderline D/C students because that is where the biggest win is for them professionally. There is very little recognition in moving a decent B to an A or A*.