MINNESOTA — More than half of U.S. states are doing a terrible job keeping dangerous levels of lead out of school drinking supplies, according to a new report. Minnesota received a letter grade of "F" in the "Get the Lead Out" report from public policy researchers, who said most U.S. schools have at least some lead in their water pipes, plumbing and fixtures.

The 22 states that received "F" grades include Michigan, where the long-running Flint water crisis brought national awareness to problems children experience when they're exposed to lead. Residents of that city of about 100,000 began complaining of discolored and particle-filled water in 2014, but it was more than two years later that the city began replacing its water lines. That project should be completed this year. A red-flag word for pediatricians, exposure to lead can cause a lifetime of problems for young children, from lower IQs to antisocial and violent behavior.

"The health threat of lead in schools' water deserves immediate attention from state and local policymakers for two reasons," the Environment America Research & Policy Center and the U.S. Public Interest Research Group Education Fund wrote in the report. "First, lead is highly toxic and especially damaging to children — impairing how they learn, grow, and behave," the "Get the Lead Out" report said. "So, we ought to be particularly vigilant against this health threat at schools and pre-schools, where our children spend their days learning and playing."

The report said that "in all likelihood, the confirmed cases of lead in schools' water are just the tip of the iceberg." Several states don't require schools and preschools to address the threat of lead in drinking water, according to the report, and most of the handful of states that do "follow flaws in the federal rules — relying on testing instead of prevention and using standards that allow health-threatening levels of lead to persist in our children's water at school."

Only the District of Columbia and the state of Illinois were ranked above average, with letter grades of "B+" and "B," respectively. California, New York, Oregon, Maryland, New Hampshire and New Jersey got "C+" or "C" grades, and Arizona and Massachusetts both scored a "D."

Besides Minnesota, states that got an "F" were Alabama, Colorado, Connecticut, Florida, Georgia, Indiana, Louisiana, Maine, Michigan, Montana, North Carolina, New Mexico, Ohio, Pennsylvania, Rhode Island, Tennessee, Texas, Virginia, Vermont, Washington and Wisconsin.