PHILADELPHIA (CBS) — The union representing Philadelphia teachers says recent testing revealed “alarming” levels of asbestos in the air at Alexander McClure Elementary School. The Federation of Teachers says it’s preparing a lawsuit against the Philadelphia School District for violating a best practices agreement.

A union scientist that conducted asbestos testing Tuesday says the district failed to seal a contaminated attic, leading to elevated asbestos levels in the air of several classrooms.

In response, the district conducted tests overnight that were negative.

Students returned Wednesday after more than three weeks after asbestos-contaminated pipe insulation was found.

In a statement, the school district says the union agreed on the protocol used, adding that it is “unclear as to why the union continues to raise questions.”