There is / used to be an explainer online which gives the calculation method. My reading of it, (backed by some playing around against actuals) was, an A at Advanced Higher gives 480 points, an A at Higher gives 204 points and an A at Nat 5 gives 74.
So a top performing pupil doing 7 Nat 5s in S4 and carrying on 5 to Nat 6 in S5 and then 3 to Nat 7 in S6 would end up with:
3x480 + 2x204 +2x74 = 1996
An equally bright DC might instead opt to do 5 Nat 6 in S5 and a further 4 in S6. With no unconverted Nat 5s this would give them:
9x204 = 1836
That is why 1900 is the bench mark I picked.
Otoh if a school has a low staying on rate it will have fewer points earned in S6 but this will be adjusted for in the Virtual Comparator (VC). This could be because lots of DC are going off to do apprenticeships or because they are using the college route to specialise in S6 (common in eg Arts and computing).
In your example I would be happiest with school 1 (above 1900 and beats VC) and most concerned about the middle one (below 1900 against standard VC). I would also be asking questions to better understand the third one.
If you look at the Nat 7 data you will see that very few schools have 20% doing 3 Nat 7s which is why I am sceptical about scores above 2000. The most obvious example I am aware of is that in East Renf bright DC are encouraged to do 10 rather than 7 Nat 5s and this gives the schools an extra 3x74 = 222 points to flatter their result. If they have a few DC studying extra curricular music they can get them to do Nat 6 music before S5 and extra points for ABRSM exams as well. If they have a few bilingual DC they can persuade to do a Nat 6 in their native tongue that also helps. There are also extra points for sporting and "leadership" qualifications. The bigger the cohort of privileged DC the more examples like this there will be within the "top" 20%.
I am NOT an expert in any of this and I live in hope that one day someone will validate or correct my understanding but it hasn't happened so far unfortunately - either in MN or the press. (nice to have someone to indulge my geekery with though
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