P-1 (Lupkes- driving a 2014 Peterbilt tanker/truck)) was traveling w/b on SR-12 from Toyon Hill. P-2 (Fisher - driving a Calaveras Unified School Bus ) was traveling e/b on SR-12, approaching Toyon Hill. P-1 was proceeding downhill and into a sweeping right curve. P-2 was proceeding downhill and into a sweeping left curve. It is unknown at this time which vehicle crossed over the double yellow lines. However, in doing so, both vehicle sides swiped each other. This caused P-2 to swerve to her right, causing her right side tires to travel onto the dirt shoulder. The weight of V-2 on the shoulder sucked V-2's tires into the dirt/rocks and carried the vehicle down the embankment where it overturned onto its right side.
Video recordings are being reviewed at this time to determine which party was at fault for this collision.
The roadway was closed for approximately 1 hr 15 min.