Two teenage boys have been taken to hospital and four students have been arrested after gas was sprayed at a high school in Western Australia.

Senior Sergeant John Browne said several students and teachers were complaining of breathing difficulties about 1.20pm (3.20pm AEST), which is believed to be related to "some type of noxious" gas.

The school went into lockdown and emergency services were called to the scene.

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"Initially the job was referred to DFES [the Department of Fire and Emergency Services]," Browne said.

"It was later realised it was possibility that the gas may have been pepper type spray that may have been deployed by one of the students."

Browne said police were looking for the canister and taking air samples at the school.

"I can't answer how they've obtained it," he said.

A St John Ambulance WA spokesperson said the two teenage boys were taken to Midland Hospital.