Why did the boys become frustrated? What were their plans?

Full of self-pity, and ready to sulk, Tom walks down the street away from school thinking himself to be some kind of social degenerate: "He was a forsaken, friendless boy." In his plight of loneliness, Tom decides that society has forced him to go into a life of crime. Now sobbing and utterly dismal, Tom chances to come across Joe Harper, also crying for similar reasons. Joe, whose mother had whipped him for drinking cream (a crime of which he was innocent), is in tears and has decided to lead the life of a hermit; but after Tom's persuasion, Joe agrees that a life of crime would be more desirable. So the two boys, determined to become pirates, plan to run away from home and live on Jackson's Island: an uninhabited, narrow, wooded island in the middle of the Mississippi River.