Upper Dublin High School was placed under lockdown during first period classes Monday morning following a report of a possibly armed subject at the school.
Upper Dublin Police said four male students were being held for questioning by detectives after they were seen in a hallway on school security cameras with what appeared to be a "long gun" slung over one of their shoulders.
Upper Dublin School District Superintendent Michael Pladus told parents in an email message that a "suspicious incident" had turned out to be "a student with an umbrella."
A police officer at the scene told Patch that one of the students at the school had been holding an umbrella in a gun-like manner and had been pretending to "shoot" it.
An Upper Dublin Police Department representative said by telephone that the incident was a "misunderstanding" and that students were never in danger.
Upper Dublin Police Chief Terry Thompson said the incident had been the result of "some kids in the wrong place, at the wrong time, on the wrong day." No weapons were involved, Thompson said.
Police said the students' actions were "part of a school project to simulate the human immune system protecting the body."
A number of visibly upset parents had gathered near one of the doors of the school by 9:25 a.m. A school administrator was directing parents who wanted to sign their children out of school to the school's main office, and a number of parents were doing so.
Some parents received worried calls from friends and family members who saw the police presence at the school, while other heard about the incident on the news.
Upper Dublin High School Sophomore Justin Asaraf was in gym class when the school was locked down.
He said that his teacher instructed his class to hide in the locker room, where they stayed for roughly 30 minutes.
Another student who was in the gym when the lockdown was broadcast over the loudspeakers said that she could hear running throughout the halls.
Patch editor Kyle Bagenstose contributed to this report.