The Hunting Park school had been closed since Dec. 19 for asbestos abatement.
The Philadelphia Federation of Teachers said air tests that were completed Wednesday night came back negative for asbestos. However, the union said an attic that had been sealed off with plastic for summer remediation was breached, exposing a mechanical space and several classrooms.
The PFT said School District of Philadelphia officials didn't tell the union about the breach.
District officials said in a statement that when they received additional test results Wednesday, those results were shared with the PFT. According to the district, the latest tests confirm that the building is safe to be occupied.
The statement goes on to say, "The District is unclear as to why the PFT continues to raise questions and, in some cases, seek school closures after tests confirm schools are safe."