And then there's school culture (and family culture) to consider. If the school is skeptical and unsupportive, they will put their effort into proving your child is too young for grade (as my often told story about genital mutilation assignments shows).
Yep. We're just now learning about that, first-hand.
We sent DD8 to 4th grade with the advice to keep her age to herself, because nobody would know she was only 8 unless she told them, and we want her to be able to blend in. Her homeroom teacher "outed" her within the first few days... so much for that strategy. DD thinks that led to another girl beginning to bully her. One day when DD was shoved in the hallway, she made an appeal to another teacher standing nearby, who said something to the effect of, "We don't like tattle-tales in fourth grade. If you don't like it, you can go back to third."
Leaving that story incomplete would be a disservice to the school, so, when the situation with that girl escalated to the point where DD felt ready to report it to her parents, I emailed the teachers involved, and the responses were appropriate and consistent with their self-professed policy of zero tolerance for bullying.
But yeah... great attitude, staff.