Why is Mr. Van Daan so strict about Peter's lesson?

Mr. Van Daan is concerned about his son's laziness. Peter spends all of his time with his cat, at the expense of his lessons. His father wants him to act responsibly, and his mother covers for him. 

Mr Van Dann was very concerned about Peter's lessons because he spent all his time either with his cat(Mossie) or meet Anne frequently.