Tim Kretschmer took a high-calibre weapon on March 11 from his father's bedroom and shot people at his former school and outside a mental health clinic where he had been treated.
Now his father faces 15 counts of negligent manslaughter and 13 counts of negligent bodily harm, Chief Public Prosecutor Klaus Pflieger said.
The public prosecutor's office had initially planned on an order of summary punishment for Kretschmer's father, which would usually carries a light sentence of one-year probation.
Pflieger declined to offer more details on the case.
The motive behind 17-year-old Kretschmer's actions - which included killing nine pupils, three teachers and another person outside the clinic, then hijacking a car and shooting two other people at a car dealership – remains unknown.
Kretschmer turned the gun on himself during a shoot-out with police around 30 kilometres from the school.