Regarding math, our 6th grade GT math teacher says that they found that the GT kids needed to review grade level math just before testing because otherwise the scores are poor. He said it's just a week or two on things they haven't seen for a couple of years. Maybe a test on the material learned that year would be a better comparison than testing everyone at grade level, regarless of what they learned this year.
This described one of my kids. He took a grade-level standardized test last year without any review (he was accelerated in math by two years relative to grade level). His math scores were average to slightly above. This year we did some review (not a lot) and his scores were in the 9th stanine. Sample size of one, but it makes sense.
