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I am curious, what is the importance of having really high academic achievement?
While a child is in school their achievements may be "academic achievement". Exposure to extracurricular activities helps develop the child's other interests and areas of expertise, boosting achievement. Eventually, by the time a child completes high school and is evaluated by potential colleges/universities/employers, their achievement is generally all the individual may be evaluated on. IQ is not mentioned, rather the achievements or demonstrated ability and willingness for the individual to apply their knowledge, skills, and cognitive reasoning. [Here's a recent post discussing one company's job selection criteria... IQ phobia]

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I guess my question is how often at her age is achievement consistent with ability?
In 1st grade at 7 years old, depending upon the child's birthday and local norms, she may be older than some 1st graders? In general, many children are 5 in Kindergarten, 6 in 1st grade, 7 in 2nd grade.

If her achievement test scores are compared with an AGE cohort: If more children at age 7 are receiving academic instruction at the 2nd grade level, while she is receiving 1st grade instruction, her achievement scores may be comparatively less than the cohort.

If her achievement test scores are compared with a GRDAE LEVEL cohort: If more children in her 1st grade class are 6 years old, while she is 7 years old, her achievement scores may be comparatively higher than the cohort because she has an additional year of lived experiences.

Adding a wildcard to the mix: what is learned outside of school. Gifted children often explore on their own, creating projects, asking questions, reading and learning above their formal classroom grade-level instruction, internally driven 24/7 by a deep curiosity. Their progress may also depend upon other factors such as their personality and external factors such as having an intellectually supportive and enriched environment at home.

Recognizing your child's very high aptitude and also high achievement, you may wish to research acceleration, including the Iowa Acceleration Scale (IAS), giving some preliminary thought as to whether acceleration may be an option for your child at some point.