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Her teacher has expressed the opinion that the CogAT proves that dd is not gifted and that the WISC scores are wrong.

GRRRR! CogAT is NOT an IQ test!!
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it is just further proof to dd that she is lesser than her sister, which her teacher seems to have already convinced her of.

Double GRRRR!

What if the RIAS test is high? Will they argue the vadality of that as well?
You may want to try brightminds or some other achievement excersice over the summer to see if you can have some gains in this area.
My DD6 scores in the high average on achievement test as well, but her Full Scale IQ smoked those #'s.
Not all gifted kids are good test takers. My DD6 hates to guess, which is why I believe her achievement scores are not higher. She wants to know the right answer and lacks the confidence to try to guess well. She is a sponge not a forward thinker and probally would have bombed the cogAT:)
Luckily our school district places GT kids by IQ, and will also place the high achievers if needed.
Good luck and keep us posted