The answer to this can be found in Chapter 22, which is when O-Lan tells Wang Lung the reason for his eldest son's strange behaviour as he misses more and more school and comes to behave in a way that is more and more bizarre and odd. For O-Lan, the solution to this problem is easy. When she worked in the Old Lord's house, when his sons began to act in this way, he found a female slave for them so that they could get rid of their sexual frustration. Wang Lung decides to hatch a different plan in order to help his son:
I will betroth him and we will marry him early, and there is that to be done.
Wang Lung therefore decides to try and end his eldest son's restlessness by finding a wife for him to become betrothed to and then marrying him so that he can dispense with his sexual frustrations.
Wang Lung let him go into the school in the South.