Grades test your ability to memorize, regurgitate information, and manage time. That is all. There really isn't any skill in the subject that is involved here. I got A's in all my classes, even though I am not the best at science (past the memory stuff that is). The kids that get straight A's aren't the one's who are geniuses. In fact, many geniuses get bad grades because they are only good at one or two subjects. The people who get straight A's are the students with good time management and memory skills. I full-heartedly believe that anyone can get an A.
This may sound impossible to some people. Let me clarify, time-management is a completely different concept than "having time". I was involved in everything in high school, and there isn't much of a difference now. If I look at my free time, I only have about two or three hours of dead space in my schedule every day. Time management is learning where multi-tasking is an option and seizing every opportunity to get things done.
As for memory, the best way to improve on that is figuring out how your brain works. How do you learn? I am a visual and hands-on learner. I can memorize anything if I write it on a flashcard and go through them. You just have to find that little trick that works for you.
Now that I have told you how to get better grades, it is time for me to rant. Grades are ridiculous. I have OCD and have been obsessed with getting good grades for years. I hate them. Everyone, universities and high schools, use them to judge a person's ability to learn, even though that is not what grades are. I cannot stand this wrongful judgement. Too many places judge people on their ability to do everything and not on their ability to do what they are good at. This is not okay. People wonder why there are so many depressed teens? It is because we are expected to be good at everything, even though nobody works that way. Everyone is a genius in something. Sometimes a person's genius is spread out in many subjects and others have just one. If you are amazing at sports, you are a genius at sports. You should be acknowledged for that and not considered "stupid" because you are bad at math. Like Albert Einstein says: "Everybody is a genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid."