The head of public schools in Flint, Mich., says he's unsure whether the city’s water crisis is to blame for a dramatic recent dip in student achievement.

On the most recent round of state exams, slightly more than one in 10 third graders in Flint public schools scored proficient in reading. In 2014, four in 10 did.

Many groups seized on the news as definitive evidence that lead in Flint's drinking water has caused brain damage in area children. The left-leaning Center for American Progress tweeted:

In 2015, drinking water in Flint was found to have elevated levels of lead, which is known to impair cognitive development in children, especially children under 5.

“Even the very lowest levels of exposure, we know that lead erodes a child’s IQ, shortens attention span and disrupts their behavior,” Dr. Philip Landrigan, a pediatrician and the dean for global health at the Icahn School of Medicine at Mount Sinai, told The New Republic. “We know when we do follow-up studies that children exposed when they were kids are more likely to be dyslexic, have behavioral problems and get in trouble with the law. There’s no question about that.”

The number of Flint children with elevated levels of lead doubled after the city in 2014 switched its source of drinking water from the Detroit system to the Flint River to save money. In some neighborhoods, the number of children testing for elevated lead levels more than tripled.

But lead levels in Flint children have actually declined over the past 20 years, according to data from the Michigan Department of Health and Human Services. In 1998, half of all children six years of age and under had elevated lead levels in their blood. At the peak of the 2015 crisis, roughly 5 percent of children in the city showed elevated lead levels in their blood.

Bilal Tawwab, superintendent of Flint Community Schools, says he isn’t sure the lead is the primary cause the precipitous drop in test scores.

Tawwab took over the school district in 2015, just as the lead crisis was coming to light, and one of his first actions was to “cut off the water” and begin distributing bottled water to drink in schools. But he says that shifts in how the state measures educational assessment -- and a district that has struggled for decades -- are both significant factors in the recent test scores.

“First of all the assessment changed, and you have to account for that. I knew that coming into Flint that the achievement in reading and math were challenges,” Tawwab says.

Michigan changed the exam used to test student reading in 2015 to an exam more aligned with national Common Core standards. Reading scores across the state fell as a result, from 77 percent proficiency in reading to 40 percent.

Flint public schools were struggling long before its recent water crisis. In the last three decades, the student population dropped by 85 percent, according to the district’s own figures, including a 63 percent decline in the past 10 years alone, meaning the district receives less revenue than it once did.

When Tawwab arrived, Flint faced a $21.9 million budget deficit. He's since closed that gap, but that has meant wage freezes in a place where teachers are already underpaid. Flint pays its new teachers 12 percent less than the average starting salary for a teacher in the state.

“Our greatest challenge right now is the number of teacher vacancies,” Tawwab says. “[Flint] struggles not only [to] attract them but keep them here. Our salaries are not competitive.”

Regardless of the causes, the recent drop in reading scores could presage a dire consequence for Flint students.

Third-grade reading proficiency is a leading indicator of how well a child will perform during their academic career. As educators frequently note, children from kindergarten through third grade learn to read; beyond third grade they read to learn. Children who are not proficient in reading by the third grade are four times less likely to earn a high school diploma, according to one study by the Annie E. Casey Foundation.

Flint is attempting to remedy its reading crisis.

The district is employing a multi-tiered system of support in its schools. The first tier is basic instruction, what all children receive in classrooms. The most intensive interventions include one-on-one instruction between teachers and students who need help. But that approach is stymied by the difficulty in attracting highly qualified teachers in the classroom, says Tawwab.

“When a child needs intensive intervention, that requires eyeball-to-eyeball [contact] with a high-quality teacher. And because we have so many children who need that level of intervention, we can’t have anything but a high-quality teacher in that position."

Meanwhile, water quality is still something of a problem in Flint school buildings. A new round of test results posted by the Michigan Department of Environmental Quality earlier this month found that 97 percent of water samples taken from Flint schools showed lead levels below the federal safety threshold. However, five of the nine schools in Flint had at least one sample that still showed elevated lead levels.

The district continues to distribute bottled water, and the state has agreed to flush the water system at all district buildings. A new round of tests was completed Feb. 10, but the results have yet to be released.