Comment by alexanderskates

The problem is that the prior appears to be placed over the school, rather than the individual -- ie if your school had a low proportion of high achievers in previous years, students are finding their grades marked down, almost regardless of their performance. This results in particularly high grade disparity between independent and state schools. So it is not so much the case that good students get good marks, bad students get bad marks, but rather good schools get good marks, "bad" schools get bad marks.