DS7's teachers explicitly told parents not to help with homework, not even if the kids don't understand the assignment. (If they don't understand it, they should go to the teacher and get clarification so that the teacher can get feedback about how the assignment is written.)
One of my kids' teachers gave those same directions, and we ignored her. In that case, her teaching methods were so poor, my kids wouldn't have learned any math if we had just sat on the sides and done nothing.
I suspect that many kids can't always know if they didn't understand something (they could just do it the wrong way and think everything was fine, and not grading the homework would just exacerbate that problem). I also suspect that a lot kids wouldn't go to the teacher for clarification even if they didn't understand.
ETA: I agree with aeh! The situation I'm referring to was in 5th and 6th grades
where the expectation that kids should do homework was a bit more reasonable (but only a bit!). I completely agree that in lower grades, children should be able to simply learn it in class.
