CHICAGO (AP) — Twelve Chicago Public Schools have found elevated levels of lead in their water since testing began district-wide last month — one in every five of schools tested so far.

And test results haven’t come back yet for hundreds more schools.

The results come as the district “is taking proactive steps to ensure that our children’s drinking water is safe across all schools,” said spokesman Michael Passman.

So far, 11 schools have tested above federal limits of 15 parts per billion, or .015 milligrams of lead per liter of water. The 12th school, Reilly Elementary, is being retested following concerns the test might not have been accurate.

Many children with chronic lead poisoning have been found to have aggressive behavior disorders, learning disabilities, and other behavioral problems.