They say sharing a secret with a stranger might be easier then telling someone close to you...
Benjamin is being abused by his father... His father is the only family he has left and he feels like he has nobody to turn to... not even his best friend...So when his best friend suggests that maybe talking to a stranger would feel better...that's what ben does.

After thinking ben would never see that good listening stranger...he doesn't realize that that stranger is going to be in his classes for the rest of the year because he just moved to ben's town... will the stranger keep the secret or want to help and has to share it with someone so that ben gets help out of his situation.

Hope this made sense