First of all, What is a "Straight A Student"? Is it someone who gets full marks in all subjects? Nope. Come on. Is it really even possible to get full marks on your report card? I bet you will lag behind at least somewhere. If you're good at academics, You might not be that good in sports. You might be good at both but you can't draw or sing. Right? No one can be an all-rounder. Everyone has their own pros and cons.
Most schools and universities have this very mentality that a student has to be good at everything. This so-called "perfect student" is not available anywhere except in the fantasy world.
Word "Perfect" was never created by god. We, humans, have made this word. Like so, Have you ever seen something "perfect" in nature? Every animal, plant, micro-organism, and even human beings, have very different characteristics. We, Humans, are one of the most intellectual and clever species. This is why we have diversity.
One cheat code you can use is to curry favors from your teacher. Make them happy and become the "Teacher's favorite". Honestly, you don't have to be totally dependent on this. But I suggest that you try to compliment your teacher once. Not for profit, but with all your heart. This will not only make your teachers happy but also make YOU satisfied with yourself.
So, I suggest you try to find some sports activities in which you are good at. If you can't, you can just do running and exercising on a regular basis. By doing this, you will have some stamina and this might even improve your studies. (Not to mention that you will get proper sleep.)
Don't sit in one place for hours. If you have this bad habit, you can try doing some exercises while sitting. Like so, Rotating your foot clockwise and anti-clockwise respectively. Moving them forward then backward for at least 10-15 times.
Create a proper environment for study.
Having proper light, A place where no one disturbs you, A place that is neither too silent nor too chaotic are some basic requirements for a happy study.
We have this bad habit that most of us don't notice. The place where we are playing games, in this very same place, we study, eat and even read some books which are not related to the subject. This creates confusion for your mind. Everything should be fixed. If you're studying on the table near your window, study there. Don't do other stuff like eating, doodling, or surfing the net.
When you're playing games on your PC, have a proper place like on the sofa or something. Eat dinner on your dining table (Or the place where you actually eat) Not just anywhere.
This programs your mind in a way that whenever you sit at the study table, you'll remember you have to study. When you sit on the sofa, you know that you have to play video games.
There are many ways of becoming a straight-A student. But, the key method which is almost in all of the ways is-- consistency. This thing is essential in everything that you want to do really well. In order to stay healthy, you need to be health-conscious and keep exercising not for months or years. But for your entire life. The same thing goes with studies.
Study every day. Especially the subjects in which you're not good at. Also, don't completely forget about the subjects that you already know. Sometimes, we have this over-confidence whenever we are good at a subject. We start neglecting studies thinking that we got full marks in this one so we are good enough at this (Basically it's too easy so no need to study). I repeat that's not it. You're supposed to pay attention to every single thing. That too, equally.
If you are bad at History and you're studying it for say, 1 hour, then you are also supposed to study the other subjects regularly.
Studying at the last minute is never going to make you a straight-A student. Everything goes in haste which is why you might not be able to pay attention to everything at the same time (Unless you're not a human).
Just before your session starts, find your books on the Internet. If they're not available, that's okay. You can wait for your parents to buy them. After getting them, Read the first chapter of every subject.
Now I'll tell you what would happen. If you do this, you know what is inside the chapter. Even if you didn't give it the uttermost attention. Then the teacher teaches you the chapter. When exams come you'll read it again. It's a great revision even if you don't realize this.
I bet that you will feel some improvement in yourself by trying this. Also, if you feel like you're doing this for your parent's future, your future generation, so that people don't call you an idiot, etc. I tell you, it's nonsense. You are doing this for your future, your life, your happiness. When we think in this way, you won't feel like you're doing charity by changing yourself to someone that you are not and that too, for a random person. If you are exercising, you're not doing this so that people get attracted by you like honey bees, not for some abs, not for your parents, but for your healthy body.
Changes are good as long as they make a better you. What makes you a better person? A person, a personality that makes you feel proud about yourself, a quality that makes you grow with so much confidence that you jump on your bed excitedly like a 5-year-old child, is what we call you a better YOU.