Mayor Martin J. Walsh said Thursday that two city school employees have been placed on administrative leave as officials investigate why fountains in some schools were mistakenly turned on before water testing was complete, potentially exposing children to water tainted with lead.

The Boston Water and Sewer Commission has also been mobilized to assist the school department as it grapples with the fallout after high lead levels were found in water at several schools, he said.

“We’re getting down to the root cause of what’s happening here,’’ Walsh told reporters. “It certainly concerns me that we’re putting the health of our kids at risk.’’

The two employees, who work for the BPS facilities department, were not identified.

Several parents expressed worries over the health of their children — and frustration with school officials.

“At this point, I have zero trust in what they are saying because new stuff keeps popping up,’’ said parent Tonya Tedesco.

Children as well as pregnant women and their fetuses are particularly vulnerable when it comes to lead. Exposure has been linked to IQ deficits, shortened attention spans, behavioral problems, hearing damage, stunted growth, and lowered birth weight.

Worries about lead contamination have risen nationwide after the crisis in Flint, Mich., prompting water systems and schools around the country to explore the issue, sometimes unearthing serious problems.

Natick school officials announced this week that recent testing had found high lead and copper levels in dozens of faucets and fountains in schools across the district, but said the water is now safe.

Officials in Massachusetts recently earmarked $2 million to test for lead in water at public schools statewide.

In Boston, most schools use bottled water — and have done so since the late 1980s — because of lead concerns that stem from aging plumbing.

Last year, as part of a measure to try to reduce the district’s costly reliance on bottled water, Boston launched a program to restore fountains, starting with several schools.

The project stalled recently after testing found high lead levels in some of the new fountains.

Officials had previously said none of those fountains had been turned on since being installed. But they revealed Wednesday that fountains were temporarily turned on earlier than they should have been, potentially exposing children at four schools to water with harmful amounts of lead.

Officials blamed the mistake on a lack of communication between school district employees and a third-party contractor.

School officials have said they are investigating the miscommunication as well as what caused the higher lead levels. And they are holding off on the plan to restore fountains at more schools until they develop a more-detailed plan.

The district also stepped up testing of water in schools this spring, aiming to check each building at least once annually. Previously, it had only done enough testing to meet regulatory requirements — testing two schools per year, which left some schools untested for years.

The latest round of testing revealed high lead levels in another four schools, bringing the total to eight: Boston Latin Academy, Murphy K-8 School, Kenny Elementary School, Hernandez K-8 School, Mather Elementary School, Lee K-8 School, Curley K-8 School, and Another Course to College.

The affected fountains have been shut off and replaced with bottled water, officials said. Parents with children at those schools have been notified, and results of sampling for all schools that recently underwent testing have been posted online at www.bostonpublicschools.org/water.

Schools Superintendent Tommy Chang said in a statement that department officials “are confident that all active water fountains meet state standards and are fine for students and faculty use.’’

Jerry Mogul, executive director of Massachusetts Advocates for Children, said the organization is particularly concerned about the potential effects on children in preschool programs — such as those at the Kenny School and the Murphy School.

“No child should be exposed to lead in the drinking water in schools, particularly younger children under age 5 who are also more likely to be exposed to other environmental sources of lead,’’ Mogul said in an e-mail. “For them, even levels of lead in school fountains below the EPA or state limit are a concern.’’

Odette Williamson, who has a son one of the affected schools and a daughter in another, called the situation outrageous. She said the school department should pay for children to be tested for lead exposure.

“Parents should not have to bear the financial burden,’’ Williamson said.

In a statement, BPS said that “many health care providers, such as MassHealth, cover the cost of lead testing,’’ and that its health services staff and the city’s health commission are available to answer health-related questions.

The school department also plans to hold two family information sessions next week.

John Sullivan, chief engineer for the Boston Water and Sewer Commission, where he has worked for the past four decades, said Walsh called him to help BPS.

Sullivan, who recently lent his expertise in Flint, said he’s working with the school department to develop strict testing protocols and to ensure fountains with potential problems are not activated.

“The plan’s got to be methodical so you don’t make a mistake,’’ he said.

“They had a bump in the road before I was even involved here,’’ added Sullivan. But, “everything they’re doing now — I’m very confident it will work.’’

Matt Rocheleau can be reached at matthew.rocheleau@globe.com. Follow him on Twitter @mrochele