My grandfather was a teacher. He was the headmaster of a school for boys between thirteen and eighteen. I know that he was a kind and gentle man at heart, because when I was young, he gave me presents, and seated me on his knee, and told me stories. But I believe the boys at his school were afraid of him. At school, when he walked into a room full of noisy boys, there was silence at once. When he looked at a boy with a certain look in his eyes, the boy went red in the face, and looked down at his shoes. If a boy brought him poor, careless work that was not the best the boy could do, my grandfather would pick up the boy's book and throw it across the room, shouting, "Do it all again, and bring it back to me in the morning!" If the boy was late, or if he forgot to bring the work, he had to do it again, and again, and yet again. My grandfather never forgot. He was a very different man at school from the man I saw day by day in his own home. This passage is mainly about  _   .
the author's school life how his grandfather treated him his grandfather's different characters the boys at his school were not afraid of him