Why did Laurie act the way he did in school?

The most likely reason is that Laurie is testing his boundaries in an unfamiliar setting.
At the beginning of the story, we learn that Laurie has just begun kindergarten. His mother relates that Laurie has graduated from a "sweet-voiced nursery-school tot" to a "long-trousered swaggering character." This revelation is telling. One can interpret Laurie's bravado as a sort of coping mechanism.
While kindergarten presents new opportunities for Laurie, it is also an unknown entity. Laurie likely understands that he is transitioning into new territory, where more is expected of him. Yet, because of his immaturity, Laurie has little idea of what it means to transition smoothly into new surroundings. His bad behavior is likely an attempt at maintaining a feeling of control during an unfamiliar experience.
We notice that Laurie is openly rude to the adults in his life, from his parents to his teachers. He revels in the exploits of his alter ego, Charles. As Charles, Laurie is powerful and invincible. Laurie's teacher appears to take everything in stride. She does not let Laurie get away with bad behavior. By the third or fourth week, Charles is magically transformed. He is so good that his teacher rewards him with an apple. Charles becomes known as the teacher's "helper."
So, it's clear that Laurie acted badly in the beginning of the school year as a coping mechanism.