It is true that for many children, especially those whose primary learning mode is kinesthetic, or those with ADHD, standing up helps them focus. But they block the view of the children behind them, & leave their chair sticking out in the walkway for the teacher to trip over when trying to help other children (or in middle schools, where the desk is attached to the chair, the student is blocking the walkway himself). Plus, they're hunched over.
As you can probably tell, this bugs me. Today I thought of a possible solution: lab benches. Have a few desks available for every elementary school classroom that can be set at the equivalent of counter-height for the kid. Have a stool available, so that if they want to sit, they can, but either have no back or a low enough back that the stool can be pushed completely under the desk when the child wants to stand. Put all of the "lab benches" either in the back or down one side of the room to avoid blocking other students' view of the board/teacher/screen.
I doubt anyone will take this idea seriously, but I felt like I needed to share it.