Grades are NOT everything. You will find yourself surrounded by not only teachers, but fellow classmates that will say that grades are vital for your survival.
That is far from the truth. And I know that many people have already told you this. I was given the same “wisdom”, but I was too scared of swerving away from this idea because of the immense amount of pressure coming from teachers and more importantly competitive students to succeed.
It is essential to keep a well-balanced life. It’s perfectly fine if you get a B or fail a test. You just have to learn to move on and try your best the next time! Don’t burn yourself out too much trying to be the “perfect” student. In the end, you can only achieve being a perfect student if you are yourself and proud of your abilities.
The traditional school setting is not for everyone, and if you don't do well it's not because you're not capable! Everyone is different and grades won’t be a profound indicator of your future success.