I guess it depends on the school - my kids go to a public school and I received this warning (don't correct answers) from one of my children's teachers at curriculum night. She asked us not to correct answers because parents in previous years had corrected answers, and she moved on to new material only to realize later (around testing time) that the kids had not grasped earlier information.

My oldest son's sixth grade teachers (in middle school) seemed to not like questions from kids but all the others have welcomed individual requests. Though testing is not popular with many people, the accountability measures have helped administration in my kids' elementary school, so that teachers are encouraged and required to pinpoint problems and make parents aware of problems, which can then be solved using parent, teacher, or other resources -- partly because it makes sense but also because AYP (Adequate Yearly Progress) and test scores are impacted.

Oh, I should add that the few times I've corrected my kids' homework (at their request to review), I've managed to give them bad advice; they are very quick to tell me what I got wrong and rarely ask for my help.