Author's Note: Heavily inspired by ZeroRewind's Age Lines Are Useless.
"To ensure that no underage student yields to temptation," said Dumbledore, "I will be drawing an Age Line around the Goblet of Fire once it has been placed in the entrance hall. Nobody under the age of seventeen will be able to cross this line.
"Yes, Mr. Weasley?"
Fred had his hand up. "That's just the test to make sure we really want in, right?" he said cheerfully. "Because-"
"There's plenty of ways around that," George added thoughtfully.
Dumbledore frowned at the two of them. "If you are truly so sure -"
"We are." A moment later, Fred added, "Sir."
"Then enlighten me."
Fred beamed. "Well, you've just got to get your name in, right? So the paper with your name on it is the only thing that's got to cross the line, and it isn't underage, is it?"
"Loads of ways to accomplish that," George added. "Throwing, Levitating, enchanting it to scurry across the line and hop into the Goblet on its own, attaching it to a fishing line and aiming for the Goblet, sending a bottle rocket at the Goblet..."
"Spoken like a true Gryffindor," Malfoy drawled from where he sat at the Slytherin table. "You missed the simplest method: bribing an older student to put it in." A moment later: "Ow! What was that for, Bulstrode?"
"Idiot! Don't say it out loud!"
While the Slytherins bickered, George resumed. "One million points to Slytherin for brilliance," he mimicked in a passable imitation of Professor Snape's voice, "and twenty million points from Gryffindor for suggesting foul cheating, in which my noble house would never engage.."
Fred coughed and grinned. "And we're not even mentioning the direct method, which is Levitating the Goblet across the -"
"The Goblet will be magically attached to the pedestal for just that reason, Mr. Weasley," Dumbledore said in an unusually curt voice.
Fred blinked, then rallied. "And the pedestal?"
"Will be magically attached to the floor."
"And if you, say, carved a hole in the floor, either just beneath the pedestal or just above the pedestal - technically the ceiling, but you'd be coming in from the floor of the floor above that -"
"If you could manage such a feat, despite the protections instilled in Hogwarts by the Founders, you would be summarily expelled from Hogwarts for acute vandalism."
"Really? That's a right pity, that." Fred scratched his chin. "You could also set up a Portkey to get you inside the Age Line... seems like overkill, though."
"Don't forget, House-Elves can Apparate inside Hogwarts," Lee Jordan spoke up. "So you could ask one of the Hogwarts ones to teleport you over the line. Don't have to tell them what it's for - just claim you accidentally left something inside the Line and need to pick it up."
Beside Harry, Hermione started ranting under her breath about slavery and cavalier exploitation of workers. Harry, whose idea of workers' rights was 'they can't lock you in for more than a week, because then people start to ask questions', tuned her out.
"Right you are!" Fred raised a finger. "And, on the subject of getting someone else to do it for you - they don't have to be doing it voluntarily. Understand that we would never engage in Dark magic..."
"...we were raised right, yes we were..." George filled in.
"...we understand that others may not hold to the same standards," Fred said with a pointed glance at the Slytherin table. "And there are loads of ways to do that, too. As Professor Moody's been teaching us, there's the Imperius -"
"And, if you don't fancy a stay in Azkaban, the Confundus Charm - hit them with it when they're walking up to put their name in, switch the slip with your name for the slip in their hand while they're staggered, and watch the fun," George remarked. "Or, if you're fancying a bit of brewing, just make a Love Potion."
"Oh, sweet Angelina, shall you put my name in for love of me, though I am underage?" Fred gasped, falling to one knee and clasping his hands in a beseeching gesture. She rolled her eyes and waved him off, and he clutched his chest dramatically. "O, I am slain!"
"Come to think of it, why not cut out the middleman? Can you Confound the Goblet?" George said, scratching his head and ignoring his twin's antics. "Does it have enough of a brain to Confound? Or Imperius - 'Oi, Goblet, acknowledge me as my school's Champion already'. Be right funny if it responded to Love Potion..."
"I don't think so, Mr. Weasley," Dumbledore said stiffly.
"...but you can't have everything. I take it that Veela aren't effective either?" Dumbfounded silence. "Right then, had to ask. Wait - Memory Charms! Best thing would be Memory Charms."
"Why?" Dumbledore asked, sounding rather punch-drunk.
"To make the Goblet forget there was anyone else enrolled in your school, of course." George looked contemplative. "Come to think of it, this is a tournament, isn't it? Right - you also want to make it select the worst opponents possible. Or just make it forget there are any other schools in the first place. Can't lose to opponents you don't have."
"Maybe you could make it forget it's only supposed to take one Champion from the school," Lee Jordan suggested. "We could all compete - you two, me, and Angelina. One of us is bound to win, and we could split the prize money four ways."
"If such a thing were possible," Dumbledore said curtly, "I daresay someone would already have done it. The Tournament has only been held a few hundred times."
"How do you know they haven't?" Fred asked, getting up from the floor. "They might have just been careful about it - not done anything too major, just made sure they got picked as Champion."
"Yeah, if we were seriously going to do it, we wouldn't go around saying, 'Oi, everyone, here's what we're about to do!'" George added. "Well, unless it was a double-bluff. Making it so obvious that no one would suspect us and all."
"Really, it'd be better to set another school up for cheating," Fred commented. "That way, even if they won, no one would take their win seriously."
"Except they'd still get the prize money."
"Right you are. Best to make them look like cheaters and win anyway. If we were going to cheat, that is."
"Which we definitely aren't. Now, and this is just a thought that occurred to me, d'you think the Giant Squid counts as a Hogwarts student?"
"Only human beings may compete, I'm afraid," said Dumbledore. Despite his benign smile, Harry wondered if he was considering whether Fred and George ought to be counted in that category.
"Shame, that. Seems unnecessarily bigoted. More importantly, would the Goblet remember that if you Confounded it?"
"An extremely interesting question, brother mine. And, by the way, does anyone know the Fidelius Charm? What happens if you use it on the Secret 'I cheated in the Triwizard Tournament'?"
"Oho, perhaps we see how the last few hundred cheats got away undetected."
"Say, how is the Goblet even verified to be the right one in the first place? What's to stop someone from hustling off with the real one and leaving a fake rigged up to give the desired results?"
Dumbledore, seeming to decide this conversation had entered dangerous ground, cleared his throat loudly.
"Finally, I wish to impress upon any of you wishing to compete that this tournament is not to be entered into lightly..."
"What about 'heavily'?" Fred called out. Dumbledore ignored him.
"Once a champion has been selected by the Goblet of Fire, he or she is obliged to see the tournament through to the end. The placing of your name in the goblet constitutes a binding, magical contract."
"Wait!" Fred shouted. Dumbledore seemed prepared to carry on, but he pitched his voice even louder. "'The placing of your name'? Just the placing, right? Not you doing it?" Without waiting for an answer, he continued. "Can you even do that? Sign someone else up for a binding contract? How do you go about doing that?" Harry could almost see the gears turning in Fred's head: that sort of mischief could enable some very clever - or deeply vicious - pranks. "Do they even need to know you've done it? Do you need their consent first?"
Dumbledore looked like he wanted a lawyer.
"I fear," he said, not looking fearful at all, "that it is deep and powerful magic, and, as such, well outside of what I can explain to you at this time. Regardless, in conclusion...
"There can be no change of heart once you have become a champion. Please be very sure, therefore, that you are wholeheartedly prepared to play before you drop your name into the goblet. Now, I think it is time for bed. Good night to you all."
"Wait, sir, what about -"
Author's Note: I probably missed a few dozen methods. (The funny thing is that the only part-Veela mentioned did end up a Champion... since the Goblet is vulnerable to Confounding, maybe it is vulnerable to Veela allure as well?) The Triwizard Tournament is my favorite target for fanfiction. Any time you think you've run out of things to do to it... I mean, "with it"... you can stumble across another way to abuse it. I mean "use it". You know what I mean.
"In case anyone failed to hear in the hubbub, I repeat: the Champion for Durmstrang is 'an entire box of Blast-Ended Skrewts', the Champion for Beauxbatons is 'the Giant Squid', and the Champion for Hogwarts is 'Harry Potter'. I am interested to hear what you have to say for yourselves, Mr. Weasley and Mr. Weasley."
"There must be some mistake here, sir. We didn't put Harry's name in."
"Yes, the Giant Squid was supposed to be the Hogwarts Champion. We clearly specified the Beauxbatons Champion was a harpy."