When Harry's name came out of the Goblet, Dumbledore asked him if he had asked an older student to put his name in for him.
If such a thing could've worked
a. Why didn't any of the more adventurous students think of it?
b. Why didn't Dumbledore do something to prevent that from happening? It seems like such an obvious work-around...
The more obvious question, though, is could such a thing have happened?