"How of you to say that to your own father!"
He already didn't like the fact that he is going to temporarily replace Leonardo's place, but adding to the fact that his son just said that to him, is making him hate that he is here.
"I'm just telling the truth, you know? Honesty is the best policy, that is what grandmother always says." Even though his father is bad at teaching, Zen is quite happy that his father is here to teach him and his friends. It has been a while since the last time since his father actually taught him something.
"You think I'm happy being here?" It's clear in Zack's voice that he is holding back his temper, and that somehow ignited his son's naughtiness. 'I should be here at all! I should there! In the field with Mary and the others! Why do I have to do this?' He inwardly cried.