(CNN) Voluntary blood testing began Thursday for Newark public school students, a day after officials announced that lead levels in the district's water have been elevated since at least 2012.
The Newark Public School system's early childhood programs will be the first to receive testing, Newark Public Schools Spokeswoman Dreena Whitfield told CNN.
Overall, 17,000 students attend schools with elevated lead levels. But because testing is voluntary, it's not clear yet how many will opt to undergo the blood test.
More than 60 children were tested at one school on Thursday. Results are expected to be mailed to parents within a week.
Gloria Green, the mother of a 4-year-old boy at Early Childhood School at Samuel L. Berliner, said she was concerned. Her son had been drinking out of the water fountains during his two years at the school.
A blood test the boy took one month ago turned out fine. She said she had him retested Thursday to be sure.
"He's not feeling good today," she said of her son. "Maybe it's a stomach virus. I'm hoping."
Nana Ofosu-Amaah, executive director for the school district's Office of Early Childhood, said another pre-K school will offer tests on Friday.
"We are giving parents opportunities," she said. "We're giving families one-on-one information about lead and what they can do and what support they have. This is all about getting our families information."
The action comes after testing revealed that nearly half the district's schools had issues with elevated lead levels in water, according to the Newark Public School system.
Of 66 schools, 30 locations have tested positive for lead exposure over the federal legal limit of 15 parts per billion. Lead levels as high as 558 parts per billion were measured, but most results were below 100 parts per billion.
The New Jersey Department of Environmental Protection and Newark Public Schools say they took quick action after learning the results of water testing. The school system shut off all drinking fountains last week at the affected schools and is providing alternate sources of water. The DEP also announced it has developed a new water testing regimen for all Newark Public Schools.
Flint water crisis: Did EPA keep critical memo under wraps?
In a joint statement released last week, the DEP confirmed that lead had not been found in the Newark Water Department's source water and said that lead in water often occurs when it leaches from "either lead pipes, household fixtures containing lead or lead solder."
Lead-level data from 2012 to 2015 was released by the Newark Public School System on Thursday.
The data, compiled and reviewed by the New Jersey Department of Environmental Protection, will show that "out of about 2,067 water quality samples collected between 2012 and 2015, approximately 12% reflected lead levels above the federal action level of 15 parts per billion," according to a statement issued before the data was released.
This data is similar to data compiled between December 2015 and February 2016, which shows about 10% of samples above the federal action level, according to the statement.
The worst of the samples citywide came during the 2014-2015 school year, when samples from 72% of school buildings yielded lead levels above what the EPA considers standard. The samples were taken from 68 buildings in the school district; some buildings were annexes associated with the same school.
"It is hard to imagine that a serious threat to the health of our children would have continued for so long in secrecy under a school administration chosen by representatives of the people of Newark," Mayor Ras J. Baraka said in a statement. "Two decades of state control have allowed this problem to fester. It is time for the state to return control of our schools to the people and pay for a solution that is effective and long-term."
The new blood testing comes in light of allegations by the Newark Teachers' Union and the New Jersey chapter of the Sierra Club that the school system knew about lead concerns for years without taking action.
The Sierra Club released an internal memo sent to principals, head custodians and other administrators near the beginning of the 2014 school year with instructions to flush drinking water sources daily to "reduce the risk of possible lead contamination."
"Instruct students and staff to run each fountain or drinking faucet for at least thirty (30) seconds before drinking," the memo reads. It also instructs custodians to run and flush each fountain for two minutes before the start of school each day.
"This memo is a smoking gun," said New Jersey Sierra Club director Jeff Tittel in a statement. "They would never have sent out this memo if they didn't know that there was a problem with lead and that the problem had been going on for some time," he also said.
Why lead is so dangerous for children
But the school system says it began taking precautions in 2004 in conjunction with EPA regulations and that letters of that nature have been sent out each year since 2004. Whitfield provided CNN with similar memos dated 2007 and 2008. Whitfield also said in a statement that the school system installed filters as part of its lead reduction program.
The Union and the Sierra Club allege that the maintenance of these filters is yet another misstep by the school system.
On Monday, Newark Teachers Union President John Abeigon released photographs he said show expired water filters at 10 Newark schools not included in the 30 that showed positive results for lead in the water. The images show filters with dates written on them in marker, ranging from 2009 to late 2015. Abeigon said these dates mean the filters were expired.
In Wednesday's news release, Newark Schools Superintendent Chris Cerf refuted the allegations about the filters, calling Abeigon's claims "irresponsible."
"The dates on the filter housing do not necessarily correspond to the dates of the filter replacement," Cerf said in the statement.
The Newark Teachers Union has called for Cerf's resignation, alleging that Cerf chose to conceal the lead issue from the public.
"All the while, he has aided and abetted the growth of budget-sapping corporate charters in the district even at the expense of knowingly poisoning the students and staff whose safety during the school day he is responsible for," Abeigon wrote in a statement.
How to test lead levels in your own home
The school system defends its actions. Cerf said he and his team are aggressively tackling the problem with document reviews and personnel interviews. And in Wednesday's statement, Cerf shared his own concerns with the situation.
"As a parent, I, too, find the fact that the district has identified elevated levels of lead in water in each of these past years extremely concerning," he said.