How can I stop causing mischief in the classroom, now that I am fifteen years old?

Whoever reaches the age of fifteen becomes accountable for all religious obligations (تكاليف شرعية). He must discipline himself, train himself, and accustom himself to self-control and commitment through struggle (mujāhadah) and perseverance. The Prophet, peace and blessings be upon him, said: "Knowledge is only acquired through learning," and "Whoever seeks patience, Allah will make him patient." Among the things that help in this regard are maintaining the obligatory prayers, especially praying on time and with the congregation, as Allah Almighty says: "Recite, [O Muhammad], what has been revealed to you of the Book and establish prayer. Indeed, prayer prohibits immorality and wrongdoing, and the remembrance of Allah is greater. And Allah knows that which you do." Also helpful is keeping good company and reading the biographies of the righteous.