The Philadelphia School District has backed off its initial plans to restart in-person class for the 9,000 pre-K through second grade students.

The students, who were set to return to classes on Monday, will continue virtual learning instead, the district announced Thursday.

The decision comes as negotiations between the School District and Philadelphia Federation of Teachers continue. The union has expressed safety concerns regarding teachers and staff returning to classrooms amid the coronavirus pandemic. 

"The City-led third-party mediation process between the School District of Philadelphia and the Philadelphia Federation of Teachers on the phased reopening of our city’s public schools is nearing conclusion,” a spokesperson for Mayor Kenney wrote. 

Officials said details are still being finalized and will be announced during a press conference Monday at 1 p.m. 

"We continue to work through the mediation process to ensure that when school buildings reopen, our members and the students they serve will be safe," a spokesperson for the Philadelphia Federation of Teachers told NBC10.

The district said there will be a "robust" COVID-19 testing program in which all staff would go through weekly mandatory testing if they want to be inside schools. Some students would also get a test.

"Each week, school nurses will administer rapid testing to twenty percent of all students learning in person," Philadelphia School District Superintendent Dr. William Hite said. "Students who display COVID-like symptoms during the school day will be tested immediately."

Officials said more than 22,000 invitations have been sent to teachers and staff to get the vaccine.