Welcome!

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What I'm wondering is : if the WISC shows DS as high ability (say, 120+) then how much weight does that carry as academic proof?
I'm afraid that is entirely up to your panel, but I suspect they're very focused on the progress chart from Y2 to Y6.
You could point out that you've only resorted to an EP report because you felt that in the circumstances they would need additional evidence.
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only EXS on writing sadly
This is a problem.
I also think the CATs aren't quite strong enough.
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DS was not tutored prior to the STT and I am wondering if it's advisable to state that explicitly?
How could you prove a negative?
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will extenuating circumstances dilute DS's case and should I actively down-play them?
I'm not sure about downplaying, but they do need to be concise.
The basic facts should be listed.
Normally I would hesitate to provide a commentary. It carries no weight if you were to say (for example) "
Child is extremely sensitive" - whereas if someone independent such as the headteacher writes "
Child is extremely sensitive" it does carry some weight.
Having said that, I think you do need to give your reasons for the schooling gap to show that that particular decision was soundly based.
Are we missing the transfer test score?
Just my initial thoughts.