Not if they're 'report card' grades that don't affect the rest of your life in any way, shape or form. I believe that they're just to track how you're going, so try to do your best on those ones as well :-)
Yes, if it can affect your career and future. You need to get good score in the school exams so that you can get admission in best colleges so it depends on the exam that it is necessary to score high or not.
High score can effect on your future..No school or college would like to give admission to low marks students. Practical knowledge is also important but Marks really effect