My only experience with this was when my daughter was in a Montessori school for pre-K and K. I liked the idea of a mixed-age classroom, but when my daughter was ready to move on, they wanted her to stay in K another year and be a leader for the younger kids. I had to move her to public school so she could start first grade. DD10 is now starting 6th grade with subject acceleration to the middle school for 7th grade advanced math. She would have been miserable if I hadn't pulled her out so she could move on.
If I were you, I'd ask the principal or guidance counselor at the school about what assessments they'll be doing and find out if she'll be allowed to move on to 3rd grade next year if she's ready. You may have push for assessments or assess on your own and advocate for the move forward if you feel she's ready.
