Harry Potter first met Sirius Black at the Whomping Willow, not at Hogwarts campus directly. In 'Prisoner of Azkaban', Sirius dragged Ron (with Scabbers/Peter Pettigrew) into the Shrieking Shack through the tunnel beneath the Whomping Willow. Harry and Hermione followed, leading to the confrontation where Sirius's true identity was revealed and Peter Pettigrew was exposed.