When writing your essay, you will want to bring out the fact that Amir had a chance to show loyalty but did not do so when he abandoned Hassan at the mercy of Assef and his gang. He was also particularly treacherous when he framed Hassan for the theft of his wristwatch, after which Hassan and his father decided to move away. Both of these scenes as well as the incident at the pomgranate tree happen during their late childhood and early adolescence, long before either Amir or Hassan are independent. It is evident that Amir could have chosen to behave otherwise, so I don't see why your teacher supposes that "mature" choices cannot be made while growing up but have to be an "adult thing" to do. If that were the case, then Amir could not be held responsible for how he acted when he was young.
But as circumstances go, Amir has a chance to learn from his mistakes. A rather spoilt child while growing up, he faces both precarity and hardship in America and seems to learn certain lessons from his experiences. He draws closer to his father, and he accepts Soraya for her true worth despite social stigma. Later he searches out and defends Hassan's son from the clutches of Assef, brings him back to America, and adopts him. In short, Amir makes ammends for all the bad things he did to Hassan by showing loyalty to Sohrab. The scar he gets when fending off Assef and the kite-running scene at the very end of the story symbolize the bond of friendship between Amir and Hassan re-established through Sohrab and Amir's redemption from former ways.
It would be good for you to find some quotes or excerpts from the text which you find particularly appropriate; below are two references to help you develop your ideas.