Why does the governess believe the school has made a mistake in sending Miles home in the novel, The Turn of the Screw?

Shortly after her arrival at Bly, the governess receives a letter that says Miles is being sent home from school and that he can never return. When Miles arrives at Bly, however, the governess thinks the headmaster must be mistaken, because Miles is a charming boy. Miles is a model student in the governess's classroom and sweetly denies that anything is amiss when the governess tries to question him in vague terms about what is going on.