Under federal law, the vast majority of schools don’t have to test the water flowing out of their taps and drinking fountains, and many states and districts also do not mandate water testing at schools. Even when districts do test their water, they don’t always tell parents about the problems they find.

This is not a hypothetical issue, nor a new one. Acute lead contamination has been found in school water in many cities during the past 15 years, including Los Angeles, Seattle, Baltimore and the District of Columbia.

But the problem of undetected lead in school water is receiving new attention in the aftermath of the crisis in Flint, Mich., where a switch in drinking water sources left children exposed to high levels of lead for months, both at home and at school.

“Right now there is a yawning gap in our lead-testing protocols,” Sen. Charles E. Schumer, D-N.Y., said in a statement last week when he introduced legislation that would provide $100 million in grants to help schools test drinking water for lead. “It’s disturbing that Flint may have been just the tip of the iceberg when it comes to toxic lead in our kids’ drinking water.”

Schumer was motivated in part by the discovery of lead contamination in schools in Ithaca, N.Y., where officials began providing bottled water to students last month after finding high lead levels in two buildings, including one classroom sink with lead levels of 5,000 parts per billion — hundreds of times higher than the level at which the federal government requires action.

Ithaca’s findings spurred officials in nearby Binghamton to reexamine test results from 2013; they found more than 50 taps with elevated lead in their public schools, including seven taps used for drinking water. And last week, elevated lead levels prompted New Jersey officials to shut off water fountains at nearly half the schools in Newark Public Schools, the state’s largest school system.

Officials said Tuesday that they will test 17,000 Newark children for lead exposure, starting with 2,000 toddlers who attend preschools and day care centers. They also acknowledged that Newark school leaders have been aware of the risk of high lead levels for more than a decade, and have managed the risk in part by directing custodians to flush water fountains daily.

Virginia Tech engineering professor Marc Edwards, who played a key role in bringing to light the crisis in Flint, said he believes the vast majority of the nation’s schools are not testing the water flowing out of their taps.

“I’m really much more concerned about the schools you do not hear anything about, and that have not tested, than I am about schools that have tested,” Edwards said.

“When people do the testing and they hear about the high lead, of course they’re rightly very worried. But everyone has to recognize that’s the good news: An adult did the sampling and protected kids going forward, thank goodness,” Edwards said. “As we’ve seen with Flint when it comes to lead what’s done cannot be undone, and we really need to get out there and do the testing so that future harm does not occur.”

The Government Accountability Office found in 2006 that “few schools and child care facilities have tested their water for lead.”

“In addition, no focal point exists at either the national or state level to collect and analyze test results,” the GAO wrote. “Thus, the pervasiveness of lead contamination in the drinking water at schools and child care facilities — and the need for more concerted action — is unclear.”

Schools that provide their own water via wells must test their own water every three years under federal law. That’s how the problem in Ithaca was uncovered, but parents weren’t told about the elevated lead until February, six months after the tests were conducted.

“I flipped out,” said Melissa Hoffman, the mother of a kindergartner and a fourth-grader at Caroline Elementary, where classrooms were found to have taps dispensing water with high lead levels. “I just assumed it would be safe.”

Hoffman said she refuses to live in fear for her children. “The damage is done. I just have to move forward with them and continue to support them in healthy ways, and feed them healthy foods, and do the best I can by them,” she said. But she’s angry, and feels that the school system’s approach to drinking water quality indicates a lack of seriousness about children’s health and safety.

But schools like Hoffman’s, where testing is required, account for just 8 to 11 percent of all schools.

The rest of the nation’s schools — about 90 percent — get their tap water from municipal sources that must be tested for lead under federal law. The testing happens at the water treatment plant, before the water courses through miles of plumbing and fixtures.

If those pipes and fixtures contain lead — and they often do, as lead-based pipes weren’t outlawed until 1986 — then water can become contaminated on the journey to the tap. If the water isn’t tested regularly as it comes out of the tap, there is no way to know if it is truly safe.

Edwards said that the water in a school is often more likely to be contaminated than the water in a home because schools close for long periods, leaving water sitting in the pipes. The stagnant water creates chemical and bacterial conditions that can intensify the lead problem, he said.

He said he’d like to see more schools testing their taps because it’s the right thing to do in order to protect vulnerable children, and he said he wishes the Environmental Protection Agency and Centers for Disease Control were more aggressive about pushing schools to initiate voluntary testing programs.

But even testing is no guarantee of safety, he emphasized: Lead solder in the plumbing can break off into the water, contaminating water so acutely that it measures at hazardous waste levels. But that occurs randomly and is difficult to capture in standard testing protocols.

“You’ll have these taps that I call Russian roulette taps. This is your worst nightmare,” Edwards said. “We’ve seen schools where drinking a single glass of water has the same lead exposure as eating five to 10 lead paint chips.”

Many school districts — such as Newark — have attempted to manage the risk of lead contamination by advising custodians to flush water fountains and other taps every day for a minute or two, or longer. Edwards said that approach can be helpful in the short term but has never been proven effective over the long term, in part because school staff often don’t flush as often or as regularly as they are supposed to.

“They’re always talking about how you have to run the water, run the water – but there’s nobody to run the water,” said Greg Goodrich, the parent of two children at Ithaca’s Enfield Elementary, where testing turned up high lead levels. “If you’re the first kid in the morning to drink out of the drinking fountain, you’re getting the largest dose.”

At schools built before 1986, Edwards said he would advise concerned parents to advocate for installing lead filters on water fountains and taps that children use to fill water bottles. Such filters are effective at protecting water quality, Edwards said, and offer more peace of mind than testing ever could.

Schools built between 1986 and 2014 are not entirely in the clear, carrying at least some risk of lead contamination in water because until 2014, brass fixtures were allowed to contain some lead. For schools from this time period, Edwards advises parents to advocate for a regular testing program to ensure that the water is safe.

Many states recognize that it’s important to ensure safe drinking water at school, but local authorities say they don’t have the resources for broad school-water testing programs, according to the EPA. “In the absence of additional federal funding,” the EPA wrote in a 2004 report, “it would be difficult to expand programs beyond existing efforts because state drinking water programs are already challenged by funding shortfalls.”

Edwards also played a central role in revealing exceedingly high lead levels in Washington, D.C.’s municipal water source in 2003 and 2004, and later in its public schools. He discovered through a Freedom of Information Act request that in one school, the water flowing out of one tap had more than 7,500 parts per billion of lead; the EPA calls for action when lead levels in schools are at or above 20 parts per billion in a 250-milliliter bottle.

The District of Columbia’s school system now posts the results of regular water testing online.

The results of D.C.’s recent tests show the importance of regular testing: Even years after installing filters on every tap to help resolve the lead problem in D.C., schools officials are still finding some school water fountains and taps with high levels.

Testing in December, for example, found two taps at the District’s Leckie Elementary with elevated lead, and both were turned off until filters could be installed. One tap, inside a classroom, had a lead level of 68 parts per billion; the other, in the basement, was 54 parts per billion.

In other cities that have faced lead crises in their public schools, it has often been parents who have raised concerns and pushed for change.

That was the case in Baltimore, where school officials knew of lead contamination in school water fountains in the early 1990s. A decade later, in 2003, James Williams Sr. — the father of a child who had been poisoned by lead paint — raised an alarm.

Williams told the city school board that he had visited a dozen schools where he saw children drinking from fountains that had been found to have elevated lead levels years earlier.

Williams’ report forced officials to address the lead problem, introducing regular water testing and rigorous water-flushing protocols. But in 2007, the Baltimore school system gave up, choosing instead to provide all schools with bottled water. At $675,000 per year, bottled water was cheaper than testing and remediation, officials said at the time.

“We took out and disabled every water fountain in the entire school system to make sure that we wouldn’t have an issue,” Keith Scroggins, the school system’s facilities chief, said in an interview.

But cost wasn’t the only issue, Scroggins said. There also was time — and trust.

Switching to bottled water “saved us a tremendous amount of headache because obviously people were still concerned that if we had all these issues with the piping, how could they actually know that we were actually doing the testing right?” he said. “Their children were at stake here.”

The school system promised to provide bottled water for all 190 of its buildings until those buildings could be renovated or replaced. Progress has been slow: Just six buildings have had the necessary upgrades to allow students to go back to filtered tap water. Now, Baltimore City Schools is hoping to rebuild or replace more than two dozen additional buildings over the next four years, Scroggins said.