The school is the Tacony Academy Charter High School, located at 6238 Rising Sun Avenue.
According to a letter that will be sent home with students, the threat was received just as the school day began Thursday.
Police say the threat was not against anyone specific, but a general threat of violence.
The school was immediately placed in lockdown as a precaution, and all students who had not yet entered the building were instructed to go home with an excused absence.
Police responded to the school and inspected all areas of the building.
Nothing was found and the lockdown was lifted as students were sent home.