A pen that was initially believed to be a rifle bullet caused the evacuation of Wednesday evening, police said.
What appeared to be a rifle round was found outside the school's main office at about 4 p.m., said Thursday. The item had ".338 CAL" imprinted on the casing around the primer, police said.
The primer was still intact and the item had the weight and appearance of a live round, police said.
Officials locked down the school, which was not in session but was open to sports teams at the time, police said. The Sussex County Sheriff's K-9 unit assisted with conducting a search of the building and grounds, but no other rounds or explosives were found, police said.
Officials reviewed video footage of where the item was found and discovered it fell out of a backpack while a student was running, police said. The student was identified and showed authorities at the police station that the item was a pen and not a bullet, police said.
"The bullet that would come out of the casing when fired has a pen attached to the end that would seat up against the powder," the police statement said. "Unless one pulled on the bullet portion to try and separate it from the casing, there is nothing to show that this is not a live round."