And the answer is... Neville. As promised a shout out to Vayanna, Style1234 and TealBlueEyes.
10) If you're caught having another school-wide prank war, there will be severe consequences.
Harry lay in wait behind a gleaming suit of armour eyes fixed in the mechanism above the door. He had visited Fred and George's shop in the holidays, and the twins had supplied him with a experimental prank.
It was developed from a muggle prank, a bucket of water above a door, but the twins took it further. When his victim opened the door water would pour down onto them, and the manifest into a 'water lion' which would chase them down the corridors.
His victim had been carefully selected. Ron had claimed no one could pull one over him as he grew up with two infamous pranksters, and up until now he had been untouched in the prank war.
Ron was currently talking to professor Flitwit in the classroom. When Ron had stayed behind Harry had run back to the dorm room and got the prank, setting it up.
Harry grinned as the door creaked open and water cascaded down. He stepped out to claim victory, but it wasn't Ron who greeted him, it was a very angry charms professor. Harry bolted as the professor (and water lion) followed.
The footsteps vanished into the distance, and the armour's visor crept up and a freckled boy smirked and clambered out of his hiding place.