(CNN) Drinking water is being shut off at all of Detroit's public schools because elevated levels of lead or copper were found in some of the buildings, the district's superintendent said ahead of the new school year.

The schools will use bottled water and water coolers instead when students return to class Tuesday, Superintendent Nikolai Vitti said.

Recent testing found "higher than acceptable levels ... (of) copper and/or lead" in at least one water source -- such as a fountain or sink -- at 16 of 24 schools, Vitti said in a statement released Wednesday.

And drinking water already had been shut off at 18 other schools because of previously identified water quality issues, Vitti said.

There is no evidence of elevated levels of lead or copper at more than 50 other schools, but test results there are pending, he said.