I learn for myself, not for my parents. So for me a 'bad grade' is when I didn't study and didn't do well on a test, or did do well on a test. I go to school because I want to, I don't have to go. I could have stopped going to school and 10th grade and have gotten a job, but I didn't because I have set a goal for myself. YES I do have a D on my Report card in Chemistry because we only had 2 weeks of Chemistry and I was sick for one of them and was extremely confused at what was going on through out the whole week that I was there. That, for me is acceptable because I wasn't able to learn what was on the test. I'll have to do better next time, and I'm planning on dropping chemistry next year anyways. For me, I consider a bad grade, when I didn't study or anything for a class... I didn't try my best. Of Course my parents arn't happy when I get a bad grade, but most of the time I get somewhere between 12 and 15 NP (We have a different grading system where we have 0 - 15 points. 15 being the best. It would be an A+ on your grading system, 14 points would be a straight A, 13 an A- and so on.)
You also can't be good at everything - some people are, but its extremly rare. So don't beat yourself up about a bad report card; life goes on, and you'll just have to do better next time, FOR YOU and not for your Parents. Getting good grades with help you achieve YOUR goals and dreams, thats why you should work to get good grades, not because your parents want you to. We keep getting told from our teachers that next year will be the real deal, we will get points for our Abitur, for which we need good grades. No one is making us take those Exams, except us.
BTW, our homework is never checked anymore. We have to do the homework for ourselves because we want to. Most people do their homework regularly because it really does help and improves your in-class work.