School officials made the decision after three current students and one recent graduate were shot over the past few days.
The only one who survived was a 15-year-old 10th grader who was seriously wounded in a shooting this past Monday. In fact, he was with a 15-year-old classmate who was among those killed.
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"And because of those four incidents that have taken place over the last week we're taking a day of healing to both allow my staff and our students to grieve," said Principal Le'Yondo Dunn.
SEE ALSO: Two 15-year-olds hospitalized after being shot in North Philadelphia
Police Commissioner Danielle Outlaw addressed the murder during Wednesday's bi-weekly gun violence briefing.
"No parent or loved one should have to go through the agonizing experience of burying their child," she said. "And our children should not be experiencing continued and sustained trauma the way they have been."
The number of shooting victims so far this year has risen to 1,612, marking an 11% increase over last year. The number of homicides in the city stands at 372 -- a 17% increase.
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Students and faculty at Simon Gratz are rallying around the 15-year-old survivor after losing three other members of the school community.
"Think about how tragic that is, not only for my school community but for those families. It's absolutely awful and they should not be the living experience for my students," said Dunn.
Simon Gratz Mastery Charter High School will remain closed on Thursday for Yom Kippur. It will be open on Friday with grief counselors available.