You Played the Opponent.

You think back to the games you've played in the past, and you remembered the chessboard from the common room. Rupert has a play style, you saw it: he misjudges his opponents, he sees only what he expects to see. It was how he had been duped before. That's your move, its risky, but you have to lull him into confidence.

You begin the game with a simple beginners gambit, just barely preventing him from mating you in the first few moves. You watch your pieces animatedly try to instruct you how to play better, and angrily tell them to "Just stuff it!" Rupert smiles at that. He's falling for your ruse.

Many new players overvalue pieces. Sure there are advantages to having pieces, but having too many pieces can clutter the field, and you carefully sacrifice pieces to free up more board space, feigning frustration at each loss. Over time Rupert's moves become quicker, more carefree, and you can tell that he has stopped taking you seriously. He sees you as an opponent to be disregarded.

Your opponent has taken the bait.

You manage distract him with your back row, while using your pawns to control the board. Its a risky maneuver, and likely to backfire. However, if Rupert continues to see you as a non-threat then it might just work out.

As the game drags on its clearly becoming close, and Rupert begins to see what you have done. There are no grand Checkmates here, in fact the game runs the profound risk of stalemating.

As other tables' games finish up the other students begin to gather around your game and the quiet chatter begins to unnerve and distract Rupert, who turns to hush people periodically. The game is down to a turn by turn jockeying for position, and you hear someone mutter "Is that Gaunt kid actually gonna win?" You smile, whether or not you Mate at this point you've won.

Having enough of the side chatter Rupert stands up and barks at everyone, "If you have nothing else to do start putting the chairs back, your whispering is disrupting the game! Some people are trying to--"

"Checkmate" you say softly, trying not to smile.

Rupert spins around; livid. "What!?!" his gaze sweeps across the board, "How!? When?!?" Lurching back to the Chessboard he sees your pieces celebrating, quite crudely, over the fallen body of his last bishop. He glares at you fumbling for words to voice his outrage.

"See you next week," you chirp. You hop down off of the chair, gather your robes around you and swiftly stroll out of the room. As the calls and cheers of the rest of the first years follow you.

"THIS- THIS-- Marvolo you get back here! GAUNT THIS ISN'T OVER!" roars Rupert over the laughter of the rest of the chess club.