An earlier decision to cancel classes Friday because of the threat has been rescinded, and officials say classes will go on as scheduled.
Police say the investigation began back on June 29 after a tipster pointed out an Instagram post, in which someone wrote: "On September 7 I will shoot up Upper Darby high school."
Investigators had not been able to find the person who posted the threat, so police met with school officials on Wednesday.
A decision was subsequently made to cancel classes.
However, police then received another tip about the message, which helped them track down the person who posted it.
That person has been identified as a 14-year-old boy from Upper Darby. The ninth-grade student was taken into custody later Wednesday.
School officials now say classes will go on as scheduled at Upper Darby High School on Friday, September 7.
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