I still don’t see how mechanically you contextualise a HAT score using the average gcse outcome for a school. But clearly there is a way.
The whole thing is tricky because clearly going to a terrible school is likely to unfairly lower gcse results but it does have slightly odd outcomes at the other end of the scale, especially as 9s and 8s are not distinguished. If a pupil goes to a school where 91% of pupils get A*/8/9 I don’t suppose a straight sweep of 9s will give a score of much over 0, and no amount of work can budge that.
And lest this precipitate a row, my son doesn’t have straight 9s and doesn’t go to the referenced 91% school - I just find the issue curious. And intractable at both ends of the spectrum.