My grandfather was a teacher.he was the headmaster of a school for boys

My grandfather was a teacher.he was the headmaster of a school for boys
between the ages of thirteen and eighteen.i know that he was a kind man at heart,because when i was young,he gave me presents,and seated me on his knee,and told me stories.but i am sure that the boys at his school were all afraid of him.
at school,when he walked into a classroom full of noisy boys,there would be silence at once.when he looked at a boy with a different look in his eyes,that boy went red in the face,and looked down at his shoes.if a boy brought him poor and careless
work that was not the best that boy could do,my grandfather would pick up the boy's book and threw it across the room,shouting"do it again,and bring it back first thing in the morning if the boy was late the next morning,or if he forgot to bring the work,he had to do it 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
一what did the writer's grandfather do in a school?
二why does the writer say that his grandfather was a kind man at heart?
三why did the boy go red in the face and look down at his shoes?
四why did the writer say that his grandfather was so different at xhool from home?
第三问题回答总觉得不准确！