Dirk is a classmate of Sam
's in 1997 who bullied Sam and Barry Cook
. Dean and Sam (in the present day) find out that Dirk had just watched his mother die of cancer; he later turned to drugs and alcohol and was dead himself before he graduated. Dirk's father laments that he had a rough time from other kids, and that people called him "Dirk the Jerk" -- a nickname Sam came up with after beating him in a fistfight outside the school.
It is Dirk who, in the present day, is possessing students and causing them to kill kids who bully them. His spirit is attached to the school bus, because his father, who drives the bus, has a lock of Dirk's hair in a bible he carries on the bus. When Dean burns this, Dirk's spirit is freed.