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That depends on how he knew. He suspected that Riddle had bad intentions from the start, but the other professors at Hogwarts only saw Riddle as the model student who never did anything wrong, so to them, Dumbledore's suspicions were groundless. Even if he could've somehow proven those suspicions, I don't think anyone could've done anything to stop Tom short of expelling him - his plans and ideologies all stemmed from the loveless nature of his conception and his upbringing at the orphanage. By his boyhood years, it was too late for them to do anything to change him. Sig 15:20, October 25, 2018 (UTC)