I didn't actually read the article- shame on me. But I'll comment anyway with just one point.
How are there measuring where kids actually are grade level wise? I mean, I have a piece of paper with the WJ -IV results on it that say my child is performing at a > 13 grade level almost across the board (except speed). But in real life what does that look like? Yes, he can read anything but he doesn't have the life experience to really understand high school literature. He wouldn't thrive in a high school literature class.
Math would be slightly different. He could work six years ahead, but his EF skills wouldn't keep up being placed there in and BM school.
I'm starting just now to realize that school teaches a lot more than just grade level content. It teaches a steady progression of EF skills, it builds up life/social experience. Just because a test says your child is x years ahead doesn't mean they'd actually thrive in a BM environment at that level.
