Police said they tested the 48-year-old woman shortly after she had used a 20-seater school bus to deliver children to the local school at Dalwallinu, 260 kilometres north-east of Perth, in February.
She failed a preliminary drug test and was taken to the police station for further testing.
Officers said that test showed the woman was driving while under the influence of cannabis.
It is understood a tip off from the public led police to drug test the woman.
She is due to face court in May on a charge of driving while under the influence of an illicit drug.