Two eight-year-old boys died after a car hit their Banksia Road Primary School demountable classroom on Tuesday morning.
Investigators believe the 52-year-old widow Maha Al-Shennag was leaning over the pick up the bottle after dropping off her child, nine, inside the school gates.
Ms Al-Shennag then accidentally pressed on the accelerator, sending the car launching off an object and flying into the demountable building, The Australian reported.
Her 12-year-old son was in the car at the time of the incident but he was unharmed.
There are no suspicions any drugs or alcohol were present in her system at the time and no mechanical faults assigned to the vehicle.
The horrific crash took the lives of eight-year-old best friends Andrew Encinas and Jihad Darwiche.