Three things to consider: academical achievements, emotional fitness and Physical abilities.
1. Academic achievements are the least important. If a child is gifted, no grade level would satisfy him/her. Yet skipping the growing process is a sure way to create a child with high expectations with low abilities. There is certain amount of apprenticeship going on in boring daily life, which assures him/her to be able to handle "ordinary" tasks. "MN has higher standard than NC" is an irrelevant statement. For if MN has an average ACT score of 24, while NC has 23, it would show a big difference between the two states, however it has little bearing on individual schools, let alone for an individual child. The best a parent can do is to compare the course syllabus to see if there is any gaps needing extra attention.
2. Emotional fitness. Does she really fit in with the older kids, or she likes to take a passive role, or she likes to be treated as a kid sister? As parents, you need to make sure that she really belongs by observing how her friends treat her, and how she interacts with others in group activities, etc. It is harder to get leadership roles in a higher age group. How she handles it is an important factor.
3. Physical abilities play another big role. To handle it directly relates to emotional fitness. Since very often, it is something that you cannot make up no matter what you try.
In short, even a gifted child skips 3 grades, he/she can catch up academically very easily if test scores are used to measure. But a parent needs to be concerned with other things. In this day and age, one can find so many extra after school activities for a child to do, while school time is so short (mine goes like 9:15 -- 3:45), I find that skipping is hardly necessary. For elementary students, I am more than enough satisfied if the school can get my children to organize his/her backpacks, do and hand in homework on time.
