Tim Kretschmer's father, a successful local businessman, legally kept more than a dozen weapons at his house, one of which - a 9mm Beretta pistol - was used to deadly effect by his son in the picturesque southwestern town of Winnenden in March.
Prosecutors said his father, "negligently made possible the actions of his son in that he stored the weapons ... in such a way that Tim could get his hands on a gun and a large amount of ammunition."
He has been charged with 15 counts of manslaughter, 13 counts of grievous bodily harm and breaking gun laws, the prosecution statement added.
On March 11, the masked teen burst into his former school and picked off nine fellow pupils and three teachers, mostly with expert execution-style shots to the head.
A further three people lost their lives in a dramatic chase and shoot-out with police before, cornered, Kretschmer turned the gun on himself.
It was the worst school shooting in Germany since April 2002, when 19-year-old Robert Steinhäuser, a disgruntled student from Erfurt in eastern Germany who had been expelled, killed 16 people and then himself.