School is supposed to begin at John F. Kennedy High School on Sept. 9. The school, located in the Marble Hill section of the Bronx, serves more than 1,200 students in grades 9-12.
Friday morning, workers were assessing the structural integrity of the Terrace View Avenue building.
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Mayor Bill de Blasio said alternative arrangements are being made.
"We are now going to work to make sure every student has a good and appropriate place for school," de Blasio said. "The JFK Building will not be re-opening by opening day of school, there will be more work to be done."
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Officials said the important thing is that students are in a safe place for school.
UPDATE @NYCSchools Chancellor says students @ JFK school complex in the Bronx will still start school on time despite explosion last night,— Stacey Sager (@staceysager7) August 21, 2015
The explosion on the sixth floor Thursday night injured three construction workers and caused significant damage to three floors of the building.
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One of the workers injured is in critical condition, and the two others are in serious condition.