From what the headmaster did before, we can see that she is not the kind of person who has never told a lie. It is because he has lied that he has no resistance to lying. So this time, his lies are enough to prove that he is not the kind of person who has never lied. Don't call it a lie. This is sheer nonsense.
Adults are better at lying. Since they can say such things, they have never told a lie when they are so big. I believe that everyone will tell some lies to some extent in their own lives. This is normal, because all people are like this, and adults are more likely to lie, because adults are forced to lie for this helpless reason.
Generally speaking, it is normal for adults to be forced to lie now. The headmaster even said that she had never told a lie when she was so big, so it was too much, and it was simply deceiving our cognition. So she is used to telling lies, so don't care about the headmaster's reasons, because it is normal for her to tell lies.