In the end of Harry Potter GoF, it says that Mad-Eye Moody (Mr. Crouch's son) put Harry's name in the goblet, which is how he was picked without being of age. But why did the goblet draw 2 names?
it also says that it would have to be very powerful wizard who did it as the goblet was made with the knowledge that only three schools competed, so they would have had to put a very strong baffling charm (or something like that) on it to make it forget this and recognise the 'fourth school'