How Do People Get So Smart?

How come people in my class are so smart and they get good marks in tests ,
i study the subjects after school but i still get bad marks!
the only subjects i get good marks in are Art , Music(which is a subject in our school (I.B board its supposed to be the best one. ) and Physical Education).
But in Academics im not doing good.
Is there a way i can get smart i am very commited to get smart.

I have moved to another country from NewZealand and its probably because ive missed out on alot because of the teaching differences.
Does anyone know how i can get better?