The bus had no children on board but was minutes away from picking up its first student when it collided with a blue Ford sedan at the intersection of Mallala Road and Navvy Hill Road at Korunye near Two Wells at 7.15am on Friday.
A 19-year-old passenger from Davoren Park died in hospital today "as a result of injuries sustained in the collision," South Australia Police said in a statement.
A 23-year-old woman, who was a passenger in the Ford, died at the scene.
The driver of the Ford sedan, a 22-year-old Freeling man, was arrested on Monday.
"(He was) charged with two counts of cause death by dangerous driving and one count of cause harm by dangerous driving," South Australia Police said in a statement.
He is expected to appear in Elizabeth Magistrates Court next month.
The driver of the bus was taken to the Lyell McEwin Hospital with minor injuries after the crash.
A colleague of the bus driver told 9News he was in shock but was OK.