It is often claimed that Einstein was a poor student who flunked math. This old story is false and Albert laughed after hearing about it. In fact, he had mastered calculus by high school and was able to understand the nearly incomprehensible philosophy of Immanuel Kant at age 13. Most adults I know can understand neither of those things.
It is true that he didn’t care for school, however. The Teutonic schools of his youth were all the opposites of what he needed. The strictness of the authorities, the regimental nature of the day, and the rote learning style clashed with his abstract genius and tendencies to daydream. While Einstein’s brilliance was able to survive this horrid environment, God only knows how many great minds didn’t.