I have been in a similar spot. It might not be underachievement. For us, my child had to try so hard during the day just to behave in an environment where no learning was happening. Then to have to come home and do more of the same was the final straw. It eventually went from whining about the dumb homework to all-out crying and refusing to do it.

This is when we approached the teacher, and from there, we eventually skipped a grade. I guess I should have gone to the teacher earlier.