Mr McGowan has dismissed claims Education Minister Sue Ellery caused unnecessary panic with her decision to supply students at five public schools with bottled water on their first day back this week.

"She took precautions, she did the right thing. She tried to protect children's health," the premier told 6PR radio on Thursday.

"There was a failure at some level in terms of getting the testing back on time."

Hundreds of students headed back to school on Wednesday amid revelations lead testing was incomplete at five schools and initial testing at five more had shown elevated lead levels in non-drinking water.

Two of the five schools returned clear test results overnight with three, Rapids Landing Primary School, Aveley North Primary School and Aveley Secondary College, remaining on bottled water.

Mr McGowan joined Ms Ellery in blaming the Education Department for the testing delay and its failure to inform the minister until Tuesday afternoon.

"I'm angry with the people involved, I think it's a failure," Mr McGowan said.

But Opposition Leader Mike Nahan said it was outrageous to blame the bureaucrats.

He said Ms Ellery had created fear and uncertainty in the minds of many parents.

"This issue did not come as a bolt out of the blue. The opposition raised the potential for lead in new school buildings with the government in September last year," he said.

Parliament was told at the time that lead testing would be conducted at the commissioning of every new school and the taps would be flushed at the start of every new year, Dr Nahan said.

"It is unbelievable that in the minister's weekly meetings with her department that testing of lead at the new schools was not a standing agenda item until she could be reassured the testing had happened and the results were clear," he said.

The lead issue comes after other recent education controversies, including a backflip on $23 million worth of education cuts following a public outcry and dropping plans to relocate the academically selective Perth Modern School.