The Detroit Public Schools says elevated levels of lead or copper have been found in water fixtures at 57 of 86 schools tested so far.
The district announced the latest test results Wednesday.
They showed 33 of the 52 schools tested have elevated levels of copper and/or lead.
It says it is still awaiting test results for 27 schools.
It says students and staff will continue drinking from water coolers.
District officials have decided no students at Detroit's 106 public schools should be subjected to drinking mains water until a solution can be found and the water declared safe.
School officials believe old fixtures, not the water source, may be to blame.