This is a good question. You are referring to the short story, "Split Cherry Tree." There are two main reasons way Pa (Luster Sexton) is angry.
First, Luster is a traditional man. He believes that doing chores is more important than education. This is reasonable to a certain degree, because he needs help on the farm. This is how he survives. Dave knows this. This is why Dave would rather take a whipping than stay after school and go home late.
Second, Luster views school from his day. He does not know that the world has changed and more importantly education has changed. He does not see that education is the way a person is going to move up in the world. We know that this is what Luster thinks, because when he goes to talk with Professor Herbert, and Professor Herbert shows him around, Luster begins to understand. He is changed at the end and wants Dave to get as much education as possible.