The Sea of Monsters (Percy Jackson&the Olympians, Book 2)

During school, Percy is stuck playing dodgeball against large seventh graders who turn out to be Laistrygonians. They summon explosive fireballs and attempt to kill Percy, but he is protected by Tyson, a friend, who is unharmed by the monsters' fireballs. Annabeth saves Percy by stabbing the last Laistrygonian from behind and takes Percy and Tyson back to Camp Half-Blood. They find the camp under attack because Thalia's tree, which protects the camp, has been poisoned by an unknown intruder. Chiron, the assistant director, has been fired because he is suspected of poisoning the tree, and is replaced by Tantalus, a spirit from the Fields of Punishment.