No one ever suspected the potions master. Which is exactly why the poltergeist would make sure it was the potions master who did the dirty work . . . who got framed for his endeavors. Mooching off the rich pureblood kids for monies for prank items was so epic!

Sliding down into the Slytherin Common Room – because that was where all the rich kids were, right? - Peeves made sure it was late in the night, when all were asleep. Checking the pockets . . . the poltergeist discreetly mooched galleons here or there, ensuring he left potion residue behind.