An emergency call came in around 8:30 a.m. for the school at 80 Loch Alsh Avenue in Fort Washington, Montgomery County.
The school district said in a statement it all began when school security saw video of four teen boys in a hallway with what looked to be a long gun slung over one of their shoulders. The teens were also acting suspiciously, officials said.
The school was placed on lockdown as the four students were found in their class and detained. Officers soon learned that the "gun" was actually an umbrella with a shoulder strap.
Meanwhile those suspicious actions, officials learned, were part of a school project to simulate the human immune system protecting the body.
"They would walk down the hallway and sort of simulate protection or attack mode. In all appearances it was very suspicious," said Deputy Chief Lee Benson of the Upper Dublin Twp. Police.
All four boys were still being interviewed by detectives.
No students were ever in any danger and all precautions were taken without delay, the district said.
The lockdown has been lifted.
Many shaken students left with their parents after the lockdown. Police responded with guns at the ready as students huddled together in classrooms.
"I was in a closet with, like, 20 other people, just waiting," said sophomore Jeff Super.
Students explained how the principal got on the loudspeaker and told them this was not a drill.
"Everyone was pretty much quiet. We were all, like, shoulder to shoulder, there was no room and we were all crouched there sitting quietly because we didn't know what was going to happen," said senior Marisa Saraceni.
"I wasn't crying, really, until I called my mom and I realized everything I could have lost if something really were to happen," said another student who did not want to be identified.
When parents heard, many came straight to the school, still reeling from Friday's mass murder at Sandy Hook Elementary in Newtown, Connecticut.
"My wife called me and I was here in a New York second. After Friday, it's crazy," said parent Craig Super.
Classes were still in session on Monday afternoon, though some students have gone home.
The school does go through lockdown drills on a regular basis. Police say they have even brought in actors to simulate a threat, so there was no confusion as to how to proceed during a lockdown.