I'm hoping Dottie or another test expert pops in, but when you test someone on WJ-III and WISC for a grade skip, you do NOT plop there scores into the next age/grade level; you use the scores of your kid in his grade level. I believe that your kiddo, at age 6, would be old enough for those tests.

Does your school have a copy of the IAS? Which version? If they don't, I would bring in your worksheet on your version, and ask them how they could come up with such a different result.