He tells her not to tell everyone because she told him that she was going to boast about him at school.
He tells her she wouldn't understand, why.
Well since you don't give the passage, i can only assume you want to know what tumult means. tumult means, either a loud and confusing noise especially caused by a large crowd. or confusing/disorder
I am pretty sure so that he could read fictious novels and other things like that
