It happened around 4 p.m. when both vehicles were traveling along Route 11.
Police say Jodie Laughlin, 50, was driving the bus when she put on her warning lights and stopped to unload students. That's when Terry Betz, 50, crashed his van into the back of the bus.
Betz's vehicle was pushed backwards into the front yard of a house.
Police say Betz was taken to York Regional Hospital for severe injuries.
Students inside the bus were not hurt in the crash.