Long version
While grades do not define a person's intelligence or worth, a grade is not simply nothing. It does correlate to success, good organization and good work ethic. After all, not many highschool dropouts etc become successful. This is not true is some cases, but in many cases it does. But you see, a C is an average grade, a C isn't bad. Work ethic and responsibility not only includes doing your homework, reviewing on time and etc, but the two also include going to teachers for help and good communication skills. Of course this means that you do not have to be a straight A student to be a good student, you don't even need A's and B's, having your own personal interests and values and specialty also counts. I knew a girl who have mostly B's and C's but she got into a very good art school for her art portfolio and her amazing steampunk jewelry designs. Another point is that grades are valued differently, evaluated differently and viewed differently in different cultures. For example many schools in Asia are criticized for their lack of creativity and critical thinking skills; much of the coursework is based on memorization. However in many European and North American school, critical thinking and self-development is practiced more than a set course. Also in some Asian countries such as China, grades reflect what would eventually be a person's final exam, a score that literally determines a person's life and worth, so they are taken much more seriously. I can assure you that the GaoKao exams are taken to an obsessive level of seriousness; sats look like nothing compared to the GaoKao. I believe Japan also does a similar test for High schoolers.
Although a person does not need straight A's or all A's and B's to be intelligent, grades often do reflect responsibility and work ethic. C is an average score, it is somewhat rare to have good work ethic and be responsible and still have a grade below a C. Not saying that's it's not possible, but it is hard. Work ethic and responsibility do also includes going to see a teacher for help, communication with adults etc.
TL;DR Version
Grades aren't my-life-is-going-to-end important, but grades reflect who student is as a person to some degree. Many people use "grades don't measure intelligence" as an excuse for laziness etc. Grades do not measure intelligence as a whole, but they do measure some aspects of intelligence. Grades and scoring also vary from culture to culture.
I'm sorry if anyone takes offense in this. It is only my personal opinion and I do not mean to insult anyone. Please let me know if I have anything I have sorely made a mistake on.
Wow so long, but I get what your saying and what the other person says too. I think you both are right. Grades may reflect your work ethic and character but at the same time there is more to people than what can be reflected in school work. I think grades are something one must earn for their own benefit and not something to do just to please parents. Yes there are more important things in life than good grades however that doesn't mean you shouldn't try your best; after all, decent grades can really boost confidence in one's
abilities. As long as one tries their best with what they have then there's no reason to feel any regrets. (▰˘◡˘▰)
P.S. I think parents in countries like Japan and Korea put more importance in grades than most parents in the U.S. Just think of it as a cultural norm.
Its sad how people care too much about grades cuz at the end of the day they are nothing. Someone who scored a high score on a test can simply be good at memorizing, but then ask them to apply that knowledge and they'd be at wit's end. Although I'm not saying that this applies to everybody who does well ob tests. But what I am saying is that even tests can't define a person's intelligence, much less a person's worth. Grades do not define a person. they may say how smart a person is, in a very findamental sense, but is that really all there is to a person?