NEWARK, N.J. -- The Environmental Protection Agency says that over four years, the drinking water in 431 schools nationwide was found to contain unsafe levels of lead.

On Thursday, the agency began testing Pre-K students in Newark after lead was found in the water at 30 schools.

"I think school is the first place that they should be safe at," father Tanquir Walker told CBS News. "That just goes to show they are not on top of their game."

The Newark school district has now revealed 12 percent of the water samples taken between 2012 and 2015 have also been above the federal limit of 15 parts per billion. Doctors say no level of lead is safe for children.

District administrator Valerie Wilson blames old fixtures.

"We have 67 schools. They average 82 years old," Wilson said. "The building infrastructure needs to be replaced. That is significant."

Wilson said the district has been addressing lead problems in schools' water since 2004, including adding water filters.

But the teachers' union released pictures of what it says are outdated filters -- some dated 2012.

Union president John Abeigon said the district isn't taking enough responsibility for the problem.

"I believe that they pushed it to the back burner because it was not on their to-do list," Abeigon said.

CBS News asked Wilson if she was comfortable that children have not been hurt by the higher lead levels.

"I am not a medical expert so I cannot provide that," she responded. "I don't want anybody to think the district is not concerned about that, right? But it is not a primary source of contamination for children."

District officials told CBS News they disagree that the filters the teachers' union photographed were outdated but could not say how many filters need to be replaced.

The superintendent has brought in state environmental officials to work on the problem. Children in all city schools are already drinking bottled water.