The School District of Philadelphia completed its inspection for asbestos at the school in the city's Tioga-Nicetown neighborhood.
Officials say problem areas that were discovered last week have been remediated and air quality samples are negative for asbestos.
The auditorium remains sealed off indefinitely for further inspection.
Asbestos also caused the middle school to close briefly. That building reopened to students on Monday.
Mastery Charter Schools CEO Scott Gordon released this statement on Wednesday:
"I am thrilled to share that the School District of Philadelphia's inspection at Gratz High School is complete, and all findings that needed to be remediated in order for the school to reopen are now complete. This includes air quality samples throughout the school, all of which came back negative for asbestos. Therefore, Mastery Simon Gratz High School will open for students tomorrow, Thursday, March 16th.
The School District of Philadelphia's inspectors have sealed off the auditorium as they complete that inspection. This is in part due to the size and very high ceilings in the auditorium. There are some other issues, like water damage, whose effects are also being thoroughly inspected. Out of an abundance of caution, the District has created a negative pressure environment so that there is no air flowing from the auditorium into the school. The work in the auditorium can continue while school is open.
Because the auditorium is closed until further notice, students who arrive early will gather in the cafeteria until classes begin at 8:05 a.m.
I will be onsite at the high school from 7:30 a.m. to 8:30 a.m. tomorrow to welcome your students back to school and answer any questions you may have.
We thank you for your patience during this unexpected process. Mastery would also like to thank the School District of Philadelphia for conducting such a thorough inspection and closely coordinating with us. Mastery will continue to work the School District to ensure that inspections of District buildings operated by Mastery Charter Schools are executed thoroughly and in such a manner as to minimize learning disruption."