Students who attend Benjamin Franklin High School and Science Leadership Academy will not return to school Thursday as planned, as officials await the results of tests that will reveal whether dangerous asbestos fibers remain in the building’s air.

Philadelphia School Superintendent William R. Hite Jr. announced Wednesday evening that classes were canceled for at least Thursday and Friday. Students had already missed planned school days Tuesday and Wednesday because of damaged asbestos found in the building.

“We will come back and continue to do some testing and abatement, and then run some additional tests over the weekend to determine what happens next week,” Hite said, adding that the closure was happening out of “an abundance of caution.”

Jerry Roseman, the Philadelphia Federation of Teachers’ director of environmental science, said the teachers’ union will insist that no staffer or student return to the building until all damaged asbestos is abated and removed — a process he said would likely take two to three weeks.

Roseman said the district must abate and clean two areas with known damaged asbestos, in the schools’ boiler room and in SLA’s first-floor commons, which is under construction. In addition, workers must go through the rest of the building to ensure that no asbestos fibers have been tracked into other areas, he said.

“Throughout the entire building, a comprehensive assessment of all asbestos materials must be conducted to ensure all material is undamaged and will not, cannot, become impacted and damaged by ongoing construction,” Roseman said. “This is the only way to protect the health, safety, and welfare of all occupants.”

Hite said a call will be made this weekend on whether to keep the building shut to students.

“If there’s a longer-term contingency plan, we would have to work on another place for the children to go while we finish the project at Ben Franklin-SLA," Hite said.

The $37 million project that is locating the schools in a single building at Broad and Green Streets has been fraught with problems. By Monday, Ben Franklin and SLA students will have lost seven days of classes to construction delays and asbestos testing and cleanup.

Hite, speaking at a news conference at School District headquarters, said that a grab-and-go breakfast will be available for students on Thursday and Friday, and that on Friday, staff will be outside the school building to hand out the free weekly SEPTA passes that students rely on to get to and from school.

Students will eventually have to make up missed days, Hite said — Ben Franklin students more than SLA students. All SLA students receive computers and so have had access to assignments, but Franklin students have not.

Officials are mulling how best to accommodate those make-up days, and will likely bring students in on some days now scheduled for teacher training.

Hite’s announcement came shortly after Franklin and SLA teachers hand-delivered a list of concerns to him and the Philadelphia school board Wednesday. More than 50 teachers expressed alarm at the way the situation has been handled, and asked the school system to test areas throughout the six-story building where there is asbestos, including tiles and pipe insulation.

The letter called on the district to relocate students temporarily and conduct air-quality tests as well as the tests on surfaces that may contain asbestos.

“For all of this school year and likely most of last year, students and staff on campus have been exposed, or at least have risked exposure, to chrysotile and amosite asbestos. After this unacceptable phase of exposure, the School District must act appropriately moving forward with a public, documented plan,” the teachers wrote. “Any secrecy, attempts to hurry reentry to the building, or pressure to silence employees on the part of the district will be understood as acts of endangerment."