I agree that your and ex and yourself should sit down with a pro acceleration psychologist and talk about underachievement. I skipped grade 4 seamlessly. I think for many kids it is the right choice.
I would try and deal with some social issues before the meeting. Will she have any cohorts? There were other kids accelerated in the city so it was easy for me to find kids my age when I got to high school. It may just be a year or so, depending on bday but there are many social changes in middle school years and puberty issues. It is good to make sure she has a buddy to share experiences.
Your ex's objectives may have more to do with the social stuff, from a guy's point of view. And they need to be addresssed. I found myself at 13 at parties with drinking, because I was in high school.
