School's out. Time to go. What happened to the rest of the guys?
~ Bus Driver
The unnamed Bus Driver was an accomplice of the Joker during the robbing of the Gotham National Bank. He stole a School Bus to use it as a getaway under the Joker's secret orders, presumably so they could disguise themselves within the line of School Buses as they headed home. He then ran the bus into Grumpy, accidentally killing him. A moment later after the money was loaded onto the bus, he looked around and asked Bozo (who was the Joker disguised) who was the Joker and what happened to the rest of the guys, where upon Joker shot the Bus Driver dead with his Machine Pistol.