During the maze event, Harry and Cedric Diggory reach the TriWizard cup together and agree to grab it simultaneously. The cup is actually a port key, and Harry and Cedric are transported to the Riddle family cemetery. Peter Pettigrew/Wormtail appears, and, on Voldemort's orders, kills Cedric. In a ritual using the bone of his father (Riddle Sr.), his servant's flesh (Pettigrew), and his enemy's blood (Harry), Lord Voldemort's shriveled body is restored to full power. He summons the Death Eaters and they assemble in a circle around him. Harry confronts Voldemort in a duel, but because their wands are 'brothers' neither can defeat the other. The wand streams connect and lock in priori incantatum. Shadows of Voldemort's murdered victims, including Harry's parents, spill out of Voldemorts wand. They momentarily shield Harry, giving him enough time to grab the port key and return to Hogwarts with Cedric's body. Back at Hogwarts, Mad-Eye Moody, who is really Barty Crouch, Jr. (using polyjuice potion to disguise himself) rushes a distraught Harry to his office. Crouch Jr. is a Death Eater, but before he can harm Harry, Dumbledore, Snape, and McGonnagal burst in and rescue him. Having run out of the potion, Crouch reverts to his normal form. It was Crouch Jr., who escaped from Azkaban Prison, who killed Crouch Sr. and entered Harry's name in the tournament. The real Mad Eye Moody, who has been locked in a trunk with a secret room attached, is rescued. At the end of the year, the Beaux Batons and Durmstrang students return to their own schools. Hermione bids Viktor good-bye, and they promise to write each other. Harry resigns himself to spending another summer at the Dursleys.
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Answer: It would have looked too suspicious. Voldemort's plan relied on secrecy, and in his position impersonating Moody, Crouch needed to make sure no one discovered him before Voldemort returned to his physical body. Disappearing from school with Harry would have blatantly tipped off Dumbledore.
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