In The Dark Knight, after the Joker blows up the hospital, he gets on a school bus which is then seen driving away as the hospital explodes. It's later mentioned that a school bus is missing with 50 people (presumably this same bus Joker got on). Whatever happened to this bus? Did they find the missing people? Or was this never mentioned again in the movie?
What happened to the bus?
The Joker hijacked it. It was his getaway vehicle, the same way he used the school bus when he robbed the bank. You can briefly see one of his henchmen grabbing the reporter (Mike Engel) and dragging him into the bus before the Joker boards it himself and the hospital blows up. Engel later appears in the Joker's video, so we know he kidnapped him.
What happened to the rest of the people?
They're his hostages in the Prewitt building. When Batman takes off a clown mask from a 'henchman', he realizes it's Engel. The hostages are dressed as clowns and the Joker's henchmen are dressed as doctors (the script specifies they're dressed as patients and doctors) as they know the police realized the Joker hijacked one of the buses from the hospital.