Given the level of selectivity of this school, the reality is that the only way a child would not achieve A-A in their A-levels would be if something went wrong on the day somehow. It does happen in normal times, but it would be hard for a teacher to predict to whom it happens. It just happens in normal times. The NLCS teachers were probably correct in saying all these girls were predicted A-A, they can hardly mark down 5-10% of girls to a B just because in a normal year there are always some who for some reason have a bad day.
When you read the detailed article, they didn't actually find any case of willfull mismarking, all these top grades could be justified which is not surprising given that only very able girls attend this school, especially in Sixth Form!
A close friend's daughter just left NLCS and is now enjoying her time at a US Ivy League school. She is happy, polite, well adjusted, very impressive, has always been happy at the school and has a lovely group of friends. I do agree that the level of criticisim of the school might be slightly overdone here and in the press.