The incident in Perth's north at Atlantis Beach Baptist College on Wednesday is understood to be the first firearms episode at a WA school.

Police arrested a 15-year-old boy at the scene about 11.50am after he allegedly fired three rounds, with one shot hitting the inside of a building.

Lacey, a student at the school, was on her way to a maths class when the first shot rang out and the school was plunged into lockdown.

"I saw a few students getting told off for saying there was an active shoot outside the school, in the parking lot," she told ABC News on Thursday

"I was confused because I didn't think they would lie about this sort of thing since it is a serious thing."

A notification on her phone from the school's administration soon confirmed the students were telling the truth.

"I couldn't believe it was happening. I wouldn't expect something like this to happen in Perth. It's crazy," Lacey said.

"I was scared, I was nervous, I was sad. There were a lot of emotions flowing around with other students."

She said she and the other students hid in a storage room at the back of their class.

"There were doors on either side with two male teachers that were either side of us and we were all cramped up in there," she said.

Lacey said the group waited in the storage area for about 30 minutes until confirmation was received that police had taken the boy, who allegedly fired the shots, into custody.

Premier Mark McGowan said it was "unthinkable" that a US-style school shooting could have happened at a WA school.

"We are very lucky no one was killed," he told reporters during a visit to the school.

"The bullets went close. It's a terrible, terrible thing and it could have been so much worse.

"This is extraordinary stuff. This is the sort of thing you see occurring in the deep south of America."

Mr McGowan said work was already under way to "dramatically reform the state's gun laws".

"There are 360,000 guns in WA (and) that's way too many," he said.

"We're going to deal with it ... and that means there will be tougher controls.

"Whatever we do as a country, never go down the pathway that America has ... with thousands and thousands of people every year getting killed by guns."

Officers took the teen into custody without incident. Two rifles, .243 and .22 calibre, were found in a car at the scene.

The weapons are understood to be registered to the boy's father. No charges have been laid and investigations continue.

"If you do own firearms, you have a responsibility to keep them in a gun safe and make sure the keys are not accessible," Mr McGowan urged.

Atlantis Beach Baptist College will reopen on Friday.

The changes to WA's gun laws are expected to be rolled out later in the year.

It forced the school, which remains closed, into lockdown, with teachers ordering students to hide as they locked classroom doors.

Australian Associated Press