PHILADELPHIA — A Philadelphia elementary school remains closed as the district works to clean up a mold problem that the teachers union alleges is long-standing.

The Philadelphia school district says it quickly shut down John B. Kelly Elementary School after finding mold in several classrooms on Wednesday. reports that the teachers union says there were complaints of mold dating back to 2015. In a statement Thursday, the union said the district allowed the problem to grow worse by not taking proactive steps.

A district spokesman questioned why the union did not say something earlier if they had complaints about the school’s condition.

The district says the school will stay closed until is mold-free.